Still backing the mastery build over the haste build at the moment.
(Rexal - Lightbringer EU)
I find that it is not worth going haste for me, even though my guild needs the damage (Elegon, I'm looking @ you...) I find myself unable to maintain a perfect rotation AND assist in leading the raid, as well as focusing on other general things that tanks have to do such as avoiding boss abilities and generating threat (Not that that is much of an issue).
And because I am often assisting in leading raids, I don't have my eyes glued to my rotation and I easily make mistakes (several times I've found myself using my HP generating abilities at 5 HP for e.g, or not using SotR for an extended period of time, losing SS and not rebuffing it for awhile).
My point being that, even if the numbers say something is better than another thing, unless there is a huge difference then go for what suits YOUR playstyle and YOUR needs more than what the theorycrafters say is "better".
My example being that I am a social raider, not a hardcore raider, my guild is very middle of the pack (Killed 8/8 Hc DS 5 weeks before MoP finished for e.g, in the weeks leading up to MoP we managed to get another 12 people the title and get most of the same people the Meta too for 22 saviour of azeroths and riders of Twilight Harbingers ), We do not intent to be hardcore and don't expect players to know the ins and outs of their class like how to spell their name, therefore while I do try to do my best, I know I can not perform the haste build properly and will hold my guild back if I was to go that route.
To all the players who think Haste is the be-all end-all of specs, be mindful of other peoples playstyles and the likes, to the others who are confused over which is better. Think about what you want to achieve, what your guild is capable of, what content you are doing and what suits yourself the most, at the moment there is no 100% clear winner between Haste and Mastery, If you are confident that you can pull of the haste build then go ahead, if you are hesistant/know you aren't that awesome/prot tanking is your off spec. then stick with Mastery for now.