My lock is dual spec'd.
1st spec is demo. I use this for solo play and heroics. Haven't done too much of old world solo instances, but I've done some. Basically, RoF with FG on defensive. Anything RoF doesn't kill, the FG cleans up for me.
The only old world instance I really venture into is Stratholme, for trying to get the mount. Simple run though, so not a great example.
2nd spec is destro. This one I use for raids, due to it's higher dps than demo.
Ideally, I'd prefer to use destro for heroics, but sadly, even though the high end content was nerf'd, some tanks still fail and can't hold the agro I pull as destro.
I've never been affliction, so can't say much about it.