It's very much viable. In a sense it's superior to shadow.
http://forum.nostalrius.org/viewtopic.php?f=39&t=1270Copying from there:
http://db.vanillagaming.org/?talent#bVMhsZbxtccbVxxThat's how my build looked like at 59. When you hit 60, you'll respecc since Spirit Tap just became useless. Whether you want to keep disc/holy build for pvp and 5-mans or go for a raid healer build depends on your personal situation of course. Spirit of Redemption and Ligthwell are useless in any case.
This build did not make me very sturdy in PvP but it made questing easy and instances a breeze. You can keep even a shitty tank alive and Inner Focus+Prayer of Healing is great for keeping the party alive when the DPS fuck up the pull or you get extras - getting a 1k cost aoe heal for free every 5 mins is certainly useful.
Wand specialization increases your DPS while you're not healing - remember the 5 second rule - and helps avoid downtime when solo questing. I would basically open by casting Shield, then Holy Fire on the mob, put SWP on it, one Smite and then wand until it's dead. Shield should last until the mob is dead and thanks Spirit tap and wanding, you
never need to sit down to drink. Unless you pull more than 1 mob, of course, which you really shouldn't.
Holy Nova is useful for the big groups of non-elite mobs in Uldaman and Maraudon, plus Rank 1 is useful for flushing rogues out of stealth when you hear that "swoosh" sound near you or see them stealth.
Killing things certainly isn't fast and you do want to group up as much as possible. I did bunch of the quests in STV in a group of 3-5 and didn't really need to kill anything - I just buffed and healed my party members while they were killing mobs and Horde. Same in Tanaris. Use db.vanillagaming.org to scan out all possible dungeon quests and do them all (remember the lvl 50 class quest in ST is not available yet) because you get really good XP from them. By the time you hit 50, you should think about grinding ST and BRD to get your BiS healing gear if you intend to raid. BRD especially has some really good gear and there's like a dozen quests to do. By 55 do the same with LBRS/Scholo/Strat.
I hit 60 without doing a single quest in Silithus or EPL, neither did I grind at all. Ran ST 4 times, BRD 12 times, LBRS once, Scholo and Strat both once before hitting 60. I did do all/most quests in Felwood, Burning Steppes, WPL and Winterspring though.