Absolutely agree with the nerites! They're awesome workers. MTS are great because the keep the substrate stirred up and prevent anerobic pockets.
Pond snails on the other hand....I had them totally over run one of my tanks once. They were in the filter, they dessimated my plants, it was awful. I'm with Epic when it comes to them.
Apples snails are great too, as long as you get the briggs and not Canas (sp?) which grow to softball size.