I have played shadow since Kara and have had my ups and downs when it comes to competetive DPS. Overall i have had a great time plying a SP and will probably keep it as my main through Mists.
Yeah I struggle with my spriest (although my gear is spec'd out for disc), but it can be fun. I can either do really well on him or suck, not much in between for some reason. I'd probably play better with him if I actually used an addon to track my DOTs.
I Love my spriest. 379 ilvl equipped can pull an easy 30k-32k+ on something where you can stand still and cast like Heroic Shannox.
The only thing I didn't like was while gearing up mana conservation was a huge problem and DPS is very skill/gear based unlike some other classes (no hate intended, but my very badly geared frost dk does a lot more dps than it should)
If I can give you 1 little tip, from my experience a good opening will set you up well for the fight in terms of cooldown/dot refreshing.