Death Grip really is a double-edged sword...on the one hand, it has huge potential if used correctly, whether it be when DPSing, tanking or PvPing. On the flip side, it can also be the most annoying and inconvenient ability ever created for other players.
Not too long ago, I happened to be tanking a VP heroic run with guild. We had one pug spot, which happened to be another DK. We get to the cat people trash, the groups with the healers and so forth. So I mark a CC target on the healer, pull the melee mobs, then grip the caster into my Death and Decay. IMMEDIATELY after it lands, said caster suddenly whizzes past my face and lands out of the DnD, with the pug DK beating on it. Moments like that happen FAR too often for my liking and it really makes me realize why people wonder about the collective intelligence of the Death Knight community : /