Weeeeellll...if you are planning on healing in endgame, and that's that:
I took shadow to level 60 or so before I respeced as holy, and I had *so much fun* with it. Why deprive yourself of the joys of shadowform and mind flay? :D
I can't recommend shadow highly enough as a leveling spec. I was healing instances from level 20 on, though - I didn't want to powerlevel a healing class, I wanted to really get a feel for my abilities - and after level 50 healing in shadow spec became noticeably more difficult. My initial plan was to take shadow to 80 and respec for healing then, once I'd gotten my soloing out of the way, but I enjoyed healing parties too much, and my performance was suffering.
I limped my way to Outland and tried healing some dungeons there, but I finally gave in and respeced to holy...and love it too, but for completely different reasons.
In parties, holy is amazing. You get to focus full time on the often intricate dance of keeping five-to-twenty-five people up and fighting, and it's an adrenaline rush that's palpably different from the dps rush I've gotten when playing melee characters in other games. As shadow, you can get away with healing until you hit the BC dungeons, but after that you really need the grab-bag of heals you only get by burrowing into the holy (or disc) tree.
But DAMN I really loved soloing as shadow. You can absolutely get the job done on holy - I do my dailies and farm, and I never die outsde of Wintergrasp (holy and PvP are...iffy at best) - but it takes more time, more effort, and just...more. You bubble, you smack mob around with a few bolts of light (which don't have anything *near* the pure dps poetry of SHADOWY TENDRILS OF DARKNESS SHOOTING OUT OF YOUR HEADSPACE AND SUCKING THE MOB'S LIFE AWAY OMG!!!1!!!), you usually have to rebubble...and eventually the mob dies. There's just no there there. The killins never feel, I don't know, joyous? to me.
With shadow, solo killing was effortless and fun. You get a grab bag of heals with holy, but you get a grab bag of innovative ways to tear up mobs when you play shadow. (I think I read that in the next patch your DoTs can CRIT, which if true oh my god.)
Unless you plan on PLing your holy to 80 by running instances and raids, you're going to spend a significant chunk of the game playing solo with quests, and you may want to farm occasionally too (though with an 80 hunter on your account that probably made you lol a little). The actual dps a shadow priest metes out might even make you lol a little, as you're used to the kickass damage output of a hunter...but I think if you level and play a dedicated shadow priest for a while you'll be amazed at the nuances of the damage a shadow can do.
It's elegant damage! :D And so much fun to suck life out of mobs, I swear. It's a class unique to any I've played in any other game.
Try it. You might like it so much you'll decide to dual spec shadow/holy. I still get a pang of pure envy when I get a shadow in my parties and I watch her tearing through mobs and healing/mana-regening the party in the process with vampiric touch and vampiric embrace.
I was so set on dual specing disc, too. Now you've sent me down a shadowy memory lane, and I am missing shadow even more. :D
(I'll probably try disc and fall in crazylove with that spec too. Priest is phun bznz, yo)