They're good if you've got a "tight" tank, like mine. I'd love to put cories in my 10g, but I have no floor space for them, there'd be no dashing about, in that thing.
As far as snails go, Malasyain Trumpet Snails seem to be the best choice. I've seen ramshorn snails and apple snails, and I've read on all three varieties... but malaysian trumpet snails almost seem ideal for a tight tank that needs a little assistance with sand upkeep and waste disposal.

I'm sure there's a catch somewhere. I hope when my MTSes come in, they do better than my pond snails do.

I have the only pond snails in existance which can't breed.
No, I'm kidding. Anyone in my situation "couldn't" breed pond snails. I have lots of ghost shrimp in my tank, they eat all the pond snail eggs, so my pond snails haven't broken the big '3' yet. My two original pond snails, that were in the tank before the ghost shrimp, and the one errant pond snail that escaped mass.. uhn, snail-icide?