The way it works is this. At first before your gear level offers you a decent amount of crit, i say decent is like 21% or more in moonkin form you're gonna want mana regen talents like Intensity and Dreamstate. Once you get past that, and you're in a 25 man raid you've got the following going for you.
Near 40% crit chance on casts, replenishments and omen of clarity.
Now, as your gear progresses your intellect value rises, raising your amount of mana that you have. So the more mana you have from gear the more you return back every time you crit. Raid buffed im just over 24k mana and I'll be returning 441 mana per critical strike. So if I crit with wrath, I return more mana than i had expended. I crit with starfire I'm only spending about 100 mana per cast.
When I'm raid buffed I'm at about 29-30% base crit strike before talents and other modifiers, this includes by normal crit strike + my idol going off, which has like near 100% up time.
This is why I don't have any mana regen talents, and don't forsee myself ever needing any. There will come a point where I will have so much mana that each critical strike with starfire will cost me no mana whatsoever, and any crit strike with wrath will return back more than the actual wrath cast, and another cast after that. (I think I'm already at that point myself)
The only mp5 talent you take is omen of clarity and thats it.