Generally, any comp is viable if both players know their class well. From my experience in retail (which could be different here in), affliction lock and rsham make a decent pair since the lock dot, fear, and run while the shaman hides behind the closest pillar. However, they tend to lose often to aggro 2s team such as hunt/rogue or hunt/warr.
I've seen affli/shadow combos but they're uncommon. I don't think it's good considering that their CC's (fears) overlap each other (i'm 78.99% ((repeating of course)) sure on this). However, the combination of the two dots can bring down a person relatively quickly...if they dont have any defensive cooldowns up.