I added some nerite snails to my tank about 3 weeks ago, spurred on by George's comments. Amazing little things, my glass, pebbles and some plants are now sparkly clean! I have not had to scrape the glass ever since adding them. They require brackish water to breed, hence they don't multiply in freshwater aquariums. So far they have not eaten any plants, I have read numerous times that they won't eat live plants, so it seems this is correct so far. I would now consider some of these essential in any of my future aquariums, though I would think that otto's may be as useful, especially as they can move around easier and hence keep the larger plant leaves clear of algae.
I have a slowly growing population of some smaller snails, these can have a go at my tiger lotus sometimes, and when I see one it is usually squashed (then eaten by the fish) or removed.