There are many people that live by it and many people that curse the day the addon was made. I am, oddly enough, both. While I do not have the addon myself I do see the merits of it. As a tool only. Everyone has an argument about it, for and against it. I will delve further into it and let everyone decide for themselves before giving my opinion.
Gear VS. Skill
This always seems to be the core argument when it comes to the gearscore addon. People against it say that gear does not equal skill and people for it say that gear shows maximum potential "if" the person has the skill. They are both right.
Let the examples begin.
I recently ran with a hunter that had roughly the same gear score as me. He had 5 more, so for all intent purposes we had the same gear score. We also had the same MM spec. So here we have a perfect example to use for gear vs. skill. I pulled 10K DPS on a single target boss and he pulled 5500. I did almost double what he did. This proves, without doubt what the people that hate the addon are saying, skill is better then gear.
+1 for Skill - Current Score: Skill 1 and Gear 0
Same hunter and same me of course. Even if I only showed one example there I beat him by a decent margin in nearly all fights, some more then others, some less, but I always won. This was a trash pull, 6 mobs, I pulled 15800 and he pulled 15100. Almost exactly what I did, within reason at least. We already showed that he was not as skilled as I was but still able to match me sometimes because he had the same gear as me.
+1 for Gear - Current Score: Skill 1 and Gear 1
Lets take 2 geared tanks I ran with. Within 100 gear score of each other. Both warriors. Both having played for over four years. The first warrior would hold anything against me. I did not even need to MD even though I do so all the time. When I see someone like that I tend to worry about it less and will not wait the extra few seconds for it to come off cool down like I do most of the time. He asked after the run if anyone came close on threat. I said that I did twice, but I keep my omen set at 80% so it is not uncommon for me and both times it was during the killing blow I got to 80%. He said, wow, these gear upgrades are working awesome. I did the same thing I always do and I have never been able to hold it with people like your DPS. So with his own words. It was his gear that made the difference. He was doing the same things he always does, he just upgraded some gear.
+1 for Gear - Current Score: Skill 1 and Gear 2
The other warrior, well, he had some serious issues. Like I said, same type of gear score, but not the same sort of results. He "seemed" to be doing everything right. I've run ICC with a killer warrior since it came out and know what to look for, for the most part. This guy "seemed" to be doing it all right but he could not even hold single mobs. I would pull off of him the second MD ended. I did not even know that was possible. Seems maybe he knew what he was doing, but not really doing it all that well. I know from tanking with my druid that sometimes it is what you do after you charge in that is more important then the original charge and positioning. If I do not drop the swipe fast enough I could lose the whole pack with a single AoE from anyone. Not really a lot of skill involved there, but it is still skill that is needed.
+1 for Skill - Current Score: Skill 2 and Gear 2
Our next example is from a healer I run with often. Holy Priest. He is geared nicely, but not uber geared. He heals ICC10 like he was healing the stocks. He makes it look easy. He always leads the charts on healing done and is near the bottom at over healing. Like I said, his gear is not uber. He is what you would say is geared for ICC10 but not excessively over geared. At best, he is healing at gear level. What he does and why he does it so well is because he is damn good at what he does. It is all skill.
+1 for Skill - Current Score: Skill 3 and Gear 2
Another holy priest I ran with would be considered over geared for ICC25 yet in an ICC10 he was all over the place. Sure, he did a solid job of healing but people with 1000 gear score less where healing the same amount he was and he over healed like crazy. I recall getting a few heals when I was at max health. Sure, thanks for the 18K heal that I did not need. Yes, I know mana is not an issue with you being you are so over geared, but getting into the habit of healing people that do not need it can be a problem should you ever face an encounter when you really need to be paying attention to the people that do need healing. His gear made him good enough to handle ICC10 but he really did lack the skills. Chalk this one up to his gear being a great healer, not him.
+1 for Gear - Current Score: Skill 3 and Gear 3
So we have seen perfect text book examples of a DPS, a Tank and a Healer. We have seen how in each case Skill and Gear can make a difference. In the end our results are exactly what I would have expected. Tied. You need to have skill and gear.
Gear can make up for some lack of skill in certain situations.
Skill can make up for some lack of gear in certain situations.
So who is right?
Both, and neither.
If you use gearscore and only gearscore to judge someone, you are an idiot. Plain and simple, there is no arguing that. Saying someone is a noob because they only have a gear score for 4500 is just as bad as thinking someone is awesome because they have a gear score of 6000. But gear score can be useful for one reason and one reason only. Judging potential.
See, on some encounters you will need people that can meet some sort of criteria. A tank with a gear score of 4500 will not be able to take 2 25K hits in a row should a worse case scenario happen and they are forced to take a few hits. So that 4500 tank, no matter how skilled they are, is not capable of doing the job.
You might run against something that will need everyone to be doing 6000 DPS and someone with a gear score of 3800, even if they are the most skilled player in the world, will not be able to do that 99.9% of the time. If the whole raid will be taking 5000 damage every 3 seconds from an area effect that is unavoidable and you can not heal enough to keep up with that then you will be of no use. Two healers of equal skill will do the same thing to handle that. One geared one will be able to keep everyone alive, the undergeared person will try but their heals will not be able to keep up with the damage being done and people will start to drop.
So as you see, gear score is important when measuring potential. Gear score shows what you are capable of doing if you know what you are doing. Of course, this is also assuming that you have the "right" gear on and not just any old gear that has the highest item level just so you can flex your epeen.
The main problem is with the asshalfs that use it do not know how to use it. The ones that think just because you have a good gear score means you are good. Remember, you can reach a gear score of 5500 just by running dungeons and never stepping a foot into a raid.
You are aware that is enough for ICC25 right?
You are saying that you will let someone into your ICC25 because he has a gearscore high enough and this person has never even once set foot into any sort of raid environment?
You can become a skilled DPS, Tank or Healer in heroics, sure, but things are a bit different in raids. All that gear shows is you have the "potential"... let me repeat that... "potential" to be good. You still need to learn the role.
Hell, I should be doing 9.5K based on my gear at any given time. I only do 7.5K on average. I still have a hell of a lot to learn. There is nothing wrong with admitting that and having a high gear score will not hide that from someone else. I've seen people with 6000 gear score doing 4K in 25 man content. I think they should ask for a refund from who ever they bought the account from on ebay. As a DPS, the worst you ever do should be around your gear score level. So if you have 6000 gear score, you better be pulling at least 6000 DPS (and should be over 10K).
Another problem is created by the people, not by gearscore itself. Like I said, gearscore is great for picking out the people that have the "potential" to pull off the raid you want to do. It helps you weed through 100 tells when you are doing ICC25 so you do not take the time taking a closer look at the people that whispered with 3800 gearscore because those people, even if they are the best players ever, do not reach the "potential" needed for that raid.
It is the people, the totally stupid people, that makes gearscore a shitty addon. Because of gearscore you will often see.
LFM Naxx, 5K+ GS
Need Tank for Heroic Randoms, must have 4K minimum.
ICC10, 6K+ achievement.
It is people looking for outrageous numbers to steamroll over things. Mind you, people with those numbers most likely do not need to be doing those things. That is the reason gearscore sucks in my mind. The stupid people that seem to use it let it get to their head.
Here is a little list for you, should you wish to use it. These numbers are numbers made to do the content easily with skilled people. They are not minimum numbers, minimum numbers will be a lot less. These are numbers that will make the content easy to do with skilled players.
Heroics: 2000 GS
Naxx 10: 2800 GS
OS 10: 2800 GS
Naxx 25: 3100 GS
OS 25: 3100 GS
EoE 10: 3200 GS
EoE 25: 3600 GS
Ulduar 10: 3600 GS
Ulduar 25: 4000 GS
Onyxia 10: 4000 GS
ToC 10: 4200 GS
Onyxia 25: 4400 GS
ToC 25: 4600 GS
ToGC 10: 4800 GS
ICC10: 5000 GS
ToGC 25: 5000 GS
ICC 25: 5200 GS
If you notice one thing, you can get well over 5200 GS just doing heroics only. Making you capable of gearing up for the top level (non heroic) content in the game to the point where you could steamroll it. Basically that means if you are not steamrolling it at those levels, you need to get more skilled yourself, get a more skilled group to run with or run with them a little more often so you can become a good team. They call that progression people. In case you did not know. And seemingly from the 5000 GS requirements I see being asked for to do Naxx, most people have no clue what progression is any more. I would love to see them do it in all blues. It can be done. It has been done, recently even.
With that all said, I will give my final opinion on the gearscore topic.
You can out gear the top level (non heroic) raid in the game just doing heroics. Being the gear is so easy to get, everyone is geared enough for it. So gearscore is not needed at all. Delete it. It is a completely useless tool. All you really need to someone to day, "I'm all T9 or better" and you are good to go.
Run some PuGs, look at the people there and not their gear score. Write down names, add them to your friends list, get the good skilled players all together without giving a crap about gear score and you will have built a team that can steamroll their way to the Lich King in no time at all. Everyone has the gear for it now... but maybe only 5% have the skill. If that many.
Lose gear score and start working on identifying skill with your own eyes instead of having an addon try and do it for you.