I decided to give the Mastery/Crit build a try last night in raid. We were going after Baleroc, and it was mentioned that this build was particularly good for Baleroc since the fight is mostly single-target heals.
The Mastery/Crit build uses the following stat priority: Intellect > Mastery > Crit > Haste > Spirit. The idea is to rely more on Holy Light and less on Divine Light. It was a little nerve-wracking to reforge away so much Spirit.
Performance-wise, the build is solid, I think. The numbers were definitely comparable to the other paladins. Additionally, the single highest source of healing done is actually from mastery shields, Illuminated Healing (something like 27% for me, followed by Beacon at 25%), and damage prevented is always better than damage healed. As well, overheals are not a complete waste, as they still give shields.
For playstyle, initially the build feels very sluggish. High haste means that all your spells are very fast, and it feels easier to react. But you get used to it after a while, and you still have Holy Shock and Word of Glory for speed.
Surprisingly, mana is not really an issue. The thing about Spirit/Haste builds is that even though you regen mana faster, you also spend mana faster. This build seems a lot less "swingy" when it comes to mana. You spend mana at a steadier rate. You still have to Judge on cooldown and use Divine Plea, like normal.
Personally, I rather like this build. It feels very solid and complements the other healers in the raid well. I'd like to see how it performs across more fights, but I think it's the build I will try for the next few weeks.