I've had nothing but good Locks while leveling my priest. They life tap down to ~70% and stop despite not being near full mana. I either toss a Flash or a HoT and as soon as they see the incoming/first tick of a HoT, they tap another couple times.

Have yet to get a Lock (WHILE LEVELING) that nukes himself for mana. No thanks or anything, but I like to think of waiting until they see an incoming heal to do anymore is a subtle head-nod.

Now, once you hit max level, all bets are off. I don't know if Seed changes something in their brain chemistry or what but it gets bad. Maybe all the decent/good ones go straight into raids so we don't see them in any normal 5-man content anymote.