I was almost to the point I wanted to change out my shadow priest for another DPS class.

But, they really shine in instances where mobs don't die instantly. I have 0 downtime right now and hardly have to heal.
I realize not all of my toons are 70 yet, so things may change.
A better breakdown of my toons:

Protection Pally, Fire Mage, Affliction Warlock w/ Imp, Shadow Priest and Resto Shammy.
I may not kill as fast as others, but I don't die or run out of mana either

ps - my level 70 gear sucks coming from holy and enhancement specs.