This week I've started leveling up a priest alt, and I've got to say, I know folks seem to be a big proponent of shadow as a leveling spec, but discipline really seems to be fitting the role well. Granted, I'm only at 24 right now, using heirloom shoulders/chest/2h/int trinket, but it seems like I can just self-bubble, Holy Fire + Penance and nuke mobs with Smite (can handle 4+ mobs at once using bubbles + Renew), and no mana issues whatsoever. I've not noticed any issues maintaining heals in 5-mans either, and that's without points in Atonement.
It looks like Disc is about to get a few good buffs with 4.0.6, this spec seems to be a good role-filler for both solo-questing and low-end dungeon content, is this normal and to be expected?