Depends on several things. If they come in uninvited, some people consider them a pest. I don't, I think they're cool little guys. There are also some snails people buy for algae cleanup(nerites), some just to watch (apple/mystery), and some to kill the 'pest' snails(assassin), and some to burrow in the substrate (MTS). There are tons of others too, but those are the most popular.