The bonus reads as follows:
"(4)Your Circle of Healing and Penance spells have a 20% chance to cause your next Flash Heal cast within 6 sec to reset the cooldown on your Circle of Healing and Penance spells."
Upon first glance, you may have the same initial reaction that I did in thinking that it'd be really cool to have my Penance CD reset after a flash heal.
After thinking about it over the last few weeks, I've come to the conclusion that there are two major problems with this set bonus.
1.) It relies on a proc in order to reset a Cooldown, on a key spell. This is a problem because any priest worth his/her salt already knows how to play well with those set cooldowns in place, and works around them.
I'm not saying that I would ever complain about my Penance CD being reset... but the problem comes when people start relying on or getting used to that Penance CD being reset.
If you get too used to that Penance CD being gone 20% of the time, and then change out some gear... your tanks or raid members may not be too pleased with you.
Personally, I've never been one to rely on procs. Like any other proc in the game, it'll be nice when it happens, but you should always plan on playing as though the proc isn't there... and just react when it is.
2.) The amount of "wasted" stats on your gear you need to go through in order to get the 4pc bonus.
As I said above, I primarily raid as Discipline. As such, it is my opinion that Spirit, as a stat, is completely worthless. (as a priest raiding current content, keep in mind)
As a Discipline priest, aside from Meditation, there are no talents that benefit from high Spirit. With the way the game is setup right now in terms of raiding, and gear, Mana is not enough of an issue to stack your spirit in order to benefit more from Meditation. You'll get spirit from other sources, there's no need to stack for it. As Discipline, it should be very rare that you ever find yourself outside of the FSR to actually benefit from straight Spirit regen. Meaning, that unlike Holy, Spirit does not increase your spellpower, nor does it noticeably increase your mana regen.
As Discipline, most of my mana regen comes from Rapture, and my PW:S being absorbed.... thus making Intellect, a much more valuable stat. Intellect increases my mana pool, making Rapture return more mana. Intellect also increases critical chance on spells, making Divine Aegis proc more often, providing more damage prevention. Intellect also passively increases your mana regen, along with spirit.
My point is, in order to receive the 4pc Tier10 bonus, you have to be wearing four of those pieces (duh). At the lowest ilevel, the priest T10 items have 406 total Spirit on the items. Granted, you don't have to wear all five pieces, but at the lowest, the gloves and shoulders, both have 71 spirit... that's a lot of what I would consider a "waste" of a stat, just for a 4pc bonus that I consider pretty mediocre.
Now, the 2pc T10 bonus is pretty nice, and worth the spirit, but the 4pc just isn't worth it for me... Especially when you look at some of the Frost Emblem items, and Ashen Verdict crafted items that don't have the spirit, but have both Haste and Crit on them... two stats I value much more highly than Spirit.