Back when the changes were announced for our mastery in 4.2, I stood up against the tide and stated that the changes were good. Maybe even amazing. Do absoloutely nothing, and you automatically get a 10%, stacking, FIFTEEN SECOND LASTING, shield for almost every heal you throw out. The fact that folks didn't jump up and cheer for this still boggles my mind.
Now, some of the top guilds are even bringing a mastery stacking Paladin to the raid, for the sole purpose of dropping gigantic shields on the tanks. Stacking full mastery, with reforging and gems, is delivering a mastery rating that gives you a shield that is 35% of your total heal. This is even higher than I anticipated, and the possibilities just boggle the mind.
Not everyone can be special
Now, this type of healer is a very specialized role. Heals will be slower, mana return will be lower, and your ability to crank out AoE healing will be greatly reduced. What you will be however is a shield generating machine. You'll probably be leaning on Holy Light a bit more, with Divine Light tossed in for those big moments, but look at the shields you'll get.
A 20k Holy Light crit will net you a 7k shield, and I think we can all agree that prevented healing is better than reactive healing (which is why we all love our Disc preist buddies). Even if you just roll Holy Lights without regard for incoming damage, your shields will continue to stack up for those moments when you really need them.
Unfortunately, you need to really be a part of a balanced heal team where everyone knows their role for this to work. This setup, while powerful, will probably have reduced results in our 10m where I'm paired with a priest and we try to two heal everything. I need to have faster heals, and as much mana and mana return as I can get. I can't afford to simply heal the MT, though I do spend more of my time doing that than healing the raid.
It's all about balance
And I'm not talking about the feathered bigfoot known as the critchicken. What I'm talking about is finding the balance in your stats, and the balance with your healing team. If you can afford to stack Mastery, then I say go for it. Personally, I'm still putting Spirit at the top - enough Haste to give me the 1.99 second HL/DL casts, and enough crit to keep Conviction up over 90% of the time. Everything else I'll put into Mastery if I can