I leveled 100-110 in about 10 hours. Their single target is really good. Obv damage output is not as good as a pure dps, but I had no problem killing things even pulling 3-4 mobs at a time without any fear of dying.
Holy - slow but i could pull 6+ mobs and never die. Single target was really slow. Normally tag mobs and then heal anyone around me and get credit. Or just pull a ton of mobs and AoE them down. Party up with a Boomy and you can just pull a whole section at a time and wreck it.
Disc - faster, could pull 4+ mobs and never die. Also I'm on a PvP server and when the silly Alliance try to jack me, they die too. Fear them into mobs, fade let them have aggro and then focus fire them down, then finish any mobs.
Shadow - fast, could pull 3+ mobs with no issues, however, sometimes would die if pulled too many or Alliance sucker punched me mid-fight.
I would say Shadow on a PvE server and Disc on a PvP server. If you go Disc spec into Divine star for another offensive spell for your rotation.
It's a breeze to tank big pulls as Disc. I regularly pulled 6-10 mobs at a time and with packs that size, I found that if anyone else was around they'd jump in to get the tag as well. Even solo though, there was no problem killing everything with PtW, DS and penance, and PW:S, SM and Masochism were awesome at keeping myself at 100%