I am a ten man raid tank and still currently going very heavy with mastery, and have given serious consideration to trying more avoidance. I have messed around a little bit in LFR with it, and found that anything under 100 percent mastery felt sort of worrisome (dropped down to 85 percent.) I swapped back to more mastery for our raid, since I was a bit nervous to change things for the real deal, but curious any other tanks have found their own personal sweet spot for Mastery, in ten mans?
I always used avoidance, because I flee haste like the plague. (I only do 25m)
So basically, whenever I can't go mastery, I go for avoidance. So far, with 562 ilvl, it only gives me about 18% crit. Which is nice, of course, but I'm still nowhere near what our monk can do, despite the class being nerfed recently.
I'm not exactly sure it's a real gain to go avoidance for pure dps, haste/crit are both better. But what I do know, I don't do 10m.
haste is hardly better, you get gcdlocked so fast (considering AMS soaking which is omnipresent in SoO or tanking more than 1 add at a time means your haste value drops drastiacally... and plain crit hast no survival benefit (while haste had "some" survival benefit in ToT) but Avoidance has (75%) Crit and is a survival stat - kind of a mixture between pure dps (crit) and tankiness.
EDIT: I like the Avoidance-build now, might switch back in the future tho depending on which bosses will be hardest.