It's a bit too situational. I guess with Darkrai you need not fear it as much, since Darkrai's going to be faster, BUT Darkrai has a lot better moves at his disposal, such as Nightmare, Dark Pulse, Spacial Rend (umad?), and assorted things like Thunderbolt. Granted you could use Dream Eater, but I wouldn't use Nightmare and Dream Eater on the same set - you run the risk of making one of them useless.
I personally prefer Nightmare, since it lets me take care of things that have absurdly high Special Defense like Blissey (Nightmare makes the opponent lose 25% of its HP every turn it remains asleep). Plus it stacks with Bad Dreams. Fun.
HOWEVER, standard competitive has a Sleep Clause in play (no more than 1 opponent's Pokemon can be put to sleep, not counting Rest), so Darkrai isn't quite as overpowering as one might think.