It's an alright card. I use it in my Galactic Tool Box deck. Crobat G slides a damage counter onto them, Darkrai G puts them to sleep. of course, he's just back up for me, and not my maine attacker. Just consistent damage with a respectable amount. becuse Honchkrow may do forty, but could also do just 20. Toxicroak could do 60, but also could do just 20. Crobat can't do any attack damage at all, Skuntank can only do 20 if you actually bother to attack with him, and weavile can do up to 70, yes, but even he can be in a position where he can only do 20. Darkrai is a constant, consistent, 50 damage, no ifs ands or buts. Sometimes you just need that consistency.
All in all, though, as a tech he's only okay, and if you're really trying to use him for a sleep deck, It would probably have been preferred if he was in a normal darkrai, that had an attack you could get to faster, + an attack that could potentially do more damage than just 50.