I've got 3 shadow priests as my DPS and they get the job done.

Having 3 of them I get to assign them one group buff each, so I'm not hammering one of their mana to zero on a rebuff. Having them all do VE is fairly effective group healing for themselves in boss fights and i just have the paladin's tank heal rotation throw an occasional flash of light on himself to keep even.

Everything in their achievements list was done as 5man PvE by myself. (links in sig) I'm a very casual player so I won't say it's a long or impressive list, but I don't think I'd have found time for doing more things with some other dps class.

One thing I definately like is the combo heal and aoe from holy nova - sure it breaks shadow form, but it makes a great burst to top everyone off and finish off the lowest mobs in a group that might have swarmed out of control until i can re-agro with the tank.