I have 5 lowbies priest specced with holy nova, it works but as said above drains the mana super quickly. with good water though it does work and I suppose with a large mana pool or later some spirit/talent it may be more viable

back in bc/wotlk I did all the instances at level with that combo, but somehow in classic so far I haven't had that much fun with them (only level 22) - I may switch to shadow and passive healing ... tbd