I've been inspecting people outside of Karazhan, and I have noticed that there appear to be three types of raiders. At least, three types when it comes to all-important question of how to socket your gear.
Type A: One Gem To Rule Them All
This person is fixated on one type of gem, and filled all their sockets with that type. They don't seem to care about socket bonuses at all. Classic examples are tanks who fill every gem slot with blue +stamina gems, or mages/warlocks who go for red +spell damage gems.
This type of player is occasionally seen on the WoW forums complaining that the sockets on gear are not the "correct" type.
Type B: Let's Match Colours
This person puts a pure blue gem in a blue socket, a pure red gem in a red socket, and a pure yellow gem in a yellow socket. They always make sure they have their socket bonuses.
Type C: Multi-colour Gems For The Win
This person uses a lot of multiple colored gems, getting lots of smaller bonuses. They'll still get their socket bonuses, but often get some effect from a socket colour not available on that piece of gear. For example, if there are blue/yellow sockets, they may go for red/blue and red/yellow gems.
I'm a Type B person myself. I like getting my socket bonuses, and there's usually one gem in each color that I like. Multi-colour just seems like too much work for me, and for some reason isn't as pleasing to my eye as matching a red gem to a red socket. For example, all my red/yellow/blue tanking sockets are +8 dodge/+8 defense/+12 stamina.
So what type of gem person are you?