Having recently leveled an alt priest, which originally used to be a 29 heal twink, from the 50's to 74 at the moment, and having healed lots of low-level instances with not so good gear (just some twink leftovers and a rare or quest reward picked up here and there), I have come to the simple conclusion that...
...holy sucks as a 5-man healing spec...
...when you're leveling. Now, I don't assume to have any knowledge of healing with a priest at level 80 in heroics, but I do know that when our priest has been discipline, and we've run heroics, me on my prot warrior, I've been pretty much invulnerable, and we've not had to sit down for a single mana break.
And that's exactly it. Having been a discipline healer, I've not had any trouble keeping my mana topped almost constantly, even in completely awful gear. After getting the talent Penance, combined with the ability Prayer of Mending, it's been a complete breeze to heal instances. I've healed Azjol-Nerub on level 70-71 with around 500 spell power, and done it without any problems. I've very rarely had to sit down to drink. If I blow away a mob and get spirit tap, I'm full mana in a matter of seconds.
As holy, I was completely lost. I ran out of mana constantly. I went through oceans of water, and I couldn't seem to heal the group well enough at boss fights. Mind you, this is with bad gear.
So, I'd have to say, if you're anywhere else than a raid, whether it's a leveling 5-man dungeon, or heroic Halls of Lightning, thank whichever god you pray to if you happen to get a discipline priest to heal you. They absolutely rock as 5-man healers.