I am currently trying to go extremely high on Mastery without reducing Crit or Haste so low I'd ruin my overall setup. This means I'll try to get to 916 Haste, ~14-15% crit, and then everything else into Mastery unless I notice my mana failing, then I'll remove some for Spirit again.
So far, results have been positive. While it's obviously not mathable (it's strength isn't providing HPS but saving lives, so comparing it to other stats in how much HPS or HPM it provides seems a bit futile), I "feel" that it's helping. In the guide I posted on the official forums, I intentionally urge people to not try compare it to the other stats, as that's misleading. Tinker with a lot of it, tinker with very little of it, decide what you like more, or keep a healthy average if you can't decide. I feel the stat is a bit weaker than it should be (but 4.0.6 buffs it slightly, so it may fit after that), but it's definitely helping me, and I ain't sure I want to drop it, even if I gain 10% crit or whatever for it. It saves people! ^_^
--(The Death of Rats, Terry Pratchett, Soul Music)