As far as questing and battlegrounds, five priests should be very decent.

I leveled 5x Disc, with a little Shadow (to Mind Flay) during RAF. And have since leveled them to 68th, for five additional Transmutes. The team will get to at least 75th, for Cataclysm Transmutes.

They're different from a lot of other compositions, which can be a good thing.

With Fear Ward, on round-robin, and the very large area of Mass Dispel, they have quite a bit of Fear protection. Not to mention you'll have five AoE fears of your own.

For the most part, spamming Holy Nova is sufficient healing for just about anything, even if it is mana intensive. As disc, I use Renew, PW: Shield, Flash Heal, Penance and Holy Nova as my heals. I suppose you could PW: Shield and Prayer of Healing (the non-talented group heal) on round-robin, for much more mana efficient AoE healing.

Five x Smite was able to kill things consistently, that 5x Lightning Bolt did not. With the shammy team, I needed to add a second cast of anything, but the priests dropped most random mobs of similar level on a single 5x volley of Smite. So while a single priest might be horrible on mana, a team of five were slightly better then 5x shaman in that regard.

DPS probably won't be that high, but you don't need all of them to go Disc either. My tools were: SW: Pain, Devouring Plague, Mind Blast, Mind Flay as the primary, and Smite as the backup although the Holy whatever (DD + Dot) is an option too. Against casters, 5x Mana Burn will be fun too.

I don't see you doing any WotLK instances (normal or heroic), without a tank. But aside from that, pretty much all content is doable, be it questing or battlegrounds.