I think they are good ones because they loose the gravel like the earthworms in the garden soil, in my aquarium I have soil sustrate mainly oak leaves when it was maturing, on th e beggining the Malayan snails release the gas bubbles aereating and avoiding the rot of the soil.
They are also a good indicators when the things go wrong, if something bad happens they climb the glass inmediatly , a quick alarm system,
PS: Excuse my grammar I'm learning to speak English...
I personally don't like them cause they seem to cover a lot of the tank at night and I also heard that they produce a lot of waste in the substrate.
I think a lot of people like them though for chruning substrate and getting rid of some algae.