Well, lesse...Clans have their ups, and their downs.
On the good side: Group of people who are serious about the game they play, they are (sometimes) quite good at the game, and when you get in a clan-squad, you know that everyone is (most likely) competent in their job/task.
On the bad side: Clans tend to make up the majority of online FPS communities, and thus if you aren't in one, then you aren't going to survive for long. They also tend to make up the vast majority of the good players, so again, you won't survive for long if you aren't in one. This also means that new and/or casual players will get frustrated with being completely curb-stomped within the first fifteen minutes of gameplay (ala Frontlines: Fuels of War).
So...I s'pose it's good in some ways, bad in a lot of others. If you're in a good, skilled clan? Awesome. If not? ...sucks to be you. Of course...I will not deny the fact that the most thrilling, pulse pounding moments in any FPS will be when you have clan-vs-clan battles. I've never had so much fun, when my clan went up against another clan in a continent-spanning skirmish during my year or two in Planetside. It was...safe to say, the most epic, and possibly one of the most memorable online-gaming moments of my life.