The site makes use of Armory data, so it stays relatively up-to-date. Builds are organized by class and by primary tree, and you can also filter for playstyle (Raid/PvP for instance). Playstyle is determined by what kind of gear the player was raiding when their data was cached, so it's not foolproof (a raider could have been wearing their PvP gear and thus counted as PvP), but it's about as good as can be done with Armory data.
As far as the information displayed goes, it seems reasonable to me. I'm a little surprised that Combat is still so dominant with Rogues, given that I've heard Mutilate got significantly buffed. And I'm very surprised that 0/61/0 is the most popular Holy Priest build; I haven't seen people talk about that one very much, and I certainly wouldn't want to be without Meditation. It does drop to only second most popular if I look at raiding Holy Priests. Are your own talent builds popular? Does TalentChic give you any ideas on how to spec?