There are a few questions I have been asked, and a few I expect to be asked because I asked them myself when I was first looking at things, so I figured I would throw together a short FAQ with some of them as there are people that might have the same questions.
1) Are garrisons required or optional?
The are both. You are required to start up your garrison when you first reach draenor. However, after you do the initial quests that set it up and pick up the quest from inside that sends you out in the world to start questing you never really need to touch your garrison again if you do not want to. I suggest, even if you do not intend to get into garrisons, just keep up on the quests from there at least so it grows with the story even if you do not build on it. Many quests will start from there and lead back to there. Might as well have your town hall, at the least, upgraded.
2) How can I visit my friends garrison?
You need to be in a group with someone to visit their garrison. You can not just visit any garrison you like. You also can not just visit any garrison of anyone in the group, only the garrison of the group leader.
You right click on your portrait and click on visit leaders garrison. When you next zone into the garrison area you will be in the leader of your groups garrison and not your own. When you wish to return to your garrison you once again right click on your portrait and choose to return to your garrison.
3) What can my friends do at my garrison?
Your friend can come to your garrison and you can fight off garrison invasions together. This is actually a very good idea to do also because it will make it easier to get a gold and golds offer nice rewards at the start of the expansion like 645 gear.
Beware however that anyone that visits your garrison has full access to it which means they can pick your herbs and mine your ore. I know, a real friend would not do that, but it is something to think about before inviting people to a group. For all you know they can click to view leaders garrison, hearth to the garrison and steal all your herbs or ore. Perhaps it is best you clean up before you join a random group unless you do not care.
But there is a good part to this. Even if you do not have an enchanters study as long as a friend does you can go to his / her garrison and disenchant using their enchanters study.
4) When does my garrison get invaded?
Whenever you want it too. It is a quest, not a system timer thing. You can start the garrison invasion whenever you wish and do it solo or with friends. I was able to get bronze, silver and gold solo in many of the invasions but they were always easier with friends and only gold gives you a bag where you can get 645 gear from it. Great for alts, if you can find a friend or two to help you get the gold.
5) Can the horde / alliance attack my garrison?
No, the garrison itself is not a PvP zone. You will never be standing around and suddenly find yourself ganked. The opposing faction can not get into your garrison and in truth I do not see that ever happening because the garrisons are phased, like the farm, where every one of them is in the same place. So if the opposing faction came and entered which one of the 3.5M garrisons would they be be flying into? Don't worry. You are safe in your garrison.
6) Do I need a profession to use the profession building?
No you do not but be aware that you will not be able to make everything a profession can make with the garrison building. Top tier items, like upgrades, best flask, weapon enchants, etc, are for people with the profession only.
Note: There is however one profession building that is kind of useless if you do not have the profession and that is the alchemy lab. While it is true you will be able to make everything outside of the top flasks and such, the work orders will be useless to you unless you have alchemy as they give the alchemical catalyst when filling them and the only things that use those are the best flasks which only an alchemist can make. An oversight by blizzard that was still on beta when it went down. With less than one week until release, I do not expect it to be corrected, but you never know.
7) Why do I need leather (fur) for tailoring?
You don't. Sumptuous fur is not leather, it is cloth. It is easy to be confused by the name. Sumptuous fur (cloth) drops from animals and the cat people on draenor. It is not like standard cloth we have gotten used to that only drops from humanoids. Thus, as it is fur, it is only dropped from animals.
8) Can I change buildings?
Absolutely. While you can not change standard buildings like the garden or mine with something else, any building you built yourself that required a decision can be changed later on. You will need to go to your architect table and drag the new building you want over the old building and pay the fee to build it, as well as wait the hour it takes to build, but any building can be replaced as long as you have the blueprints to do it.
Be aware that even if you have a level 3 building, when you replace it with something else that something else will be level 1 and you will need to build level 2 and level 3 as well.
9) How much does it cost to build the garrison to max?
This question actually has multiple answers but I will give only one of them, the most expensive version. The worst case situation, so you know the most it could ever cost you. There are buildings you can get blueprints of for free and there are 4 items you can get when questing and completing the outpost quest line in their respective zones that can be used to get a level 2 pattern for free, if you quest in all 4 zones and collect all 4 books and use all 4 books than the price will be a total of 5,000 gold less. But I am only giving you the absolute worst case scenario of building a completely level 3 garrison without using any free stuff, so you know the most you would ever conceivably need. Note: If you start replacing buildings it will cost more of course.
The absolute most you will need to build everything from the ground up once is... drum roll please...39,000 gold.
Yes, that is 39K. The number includes buying all blueprints and the cost to build then in gold. The number will be lower if you use the 4 books I mentioned to buy some blueprints, but yeah, it is a bit of a gold sink, so saving 5K gold from those books really seems worth it.
Lets hope blizzard makes a last minute change that when you get the achievements for the level 3 buildings to "learn" them instead of "unlock" them and that would save us an additional 7,000. Or how about if they made the blueprints account bound so you did not need to buy them on every single character, that would lessen the over all cost as well. Lets face it, 39K is out of reach for the vast majority of the player base.
10) Is it worth it?
That is completely subjective and it would be wrong for me to tell you yes or no. I will just tell you what I plan to do. My more active characters will surely have a completely upgraded garrison, even more than just my main characters on my main server where I have a few hundred thousand doing nothing anyway.
For the alts which I do not plan to do anything with I will only pay for and upgrade the town hall, garden, mine and salvage yard. This will make sure I have materials, follower upgrade items and the full map for all possible quests.
As I mentioned in the opening question, none of this is required. So if it is worth it is entirely up to you. I will say, if you can not afford it, do not stress it. It will not completely kill the game for you if you do not build it up to the maximum, which I believe over 90% of the player base will never be able to do anyway.
Have a question, I can try to help answer it, or even add it here?