I had an olive nerite hitchhike on some plants from my LFS. It took about a week before I learned what species of snail it was, then decided to keep it. Within a couple weeks this one dime-sized snail pretty much cleared up the algae in my 26g bowfront. Wherever it went it left little tire marks in the remaining algae because of its little forked mouthparts... I've had it eat diatoms and green spot algae. It does leave small opaque white eggs scattered everywhere, but the larvae that hatch from them don't appear to grow or really move, so I assume they die (also possible that what I'm seeing is actually some other type of baby snail). I recently moved the nerite to my otherwise empty 10g where it can eat all the diatoms it wants~
I love my nerites. One of them does a great job of cleaning all the brown algae in the tank. The other one just leaves skinny trails through the algae but rides on the shell of the other nerite 75% of the time.