I love Apple Snails. I always keep one in my tank. They do poop a lot so definitely count them towards your bioload but they are very active and always looking for leftovers. My current one is named Lucky. :)
They eat my HEALTHY wisteria when I don't feed them enough- they will devour algae wafers. Then again, mine are about the size of golfballs or bigger! So, I think they are more of a hassle than I'm willing to deal with when it comes to snails, but guests love them and I do think they are too cool cruising around the tank. They are quite beautiful and funny.
Small bioload as long as you don't feed them any extra. I give them some vegetables onces in a while, but most of the time they just eat algae, dead plants, leftover fish food and duckweed etc. Duckweed is really the only live plant they eat unless they are starving. But they do indeed poop a lot. They can adjust their buoyancy by filling their lung with gas, incredible "acrobatic" jumping/gliding between plants. Fun to watch. Can escape if the tank isn't covered. My tanks isn't covered as these escapes happens rarely and most of the time i can just put them back in the tank. I have had one escape/fall related death under the last three years.
I had one of these in my 10 gallon. Should have been plenty of food for him in there but he always insisted on nibbling on my Wisteria as well as the Purple Cardinals. He was probably a little too big for the tank though, he grew to over the size of a golf ball before I took him back to my LFS.
IF you value your aquatic plants, DO NOT buy this snail. mine devoured everything i had even when i fed it algae wafers. It even managed to somehow munch its way through all my floating plants too! they do make great tank mates though and mine was the star of my tank whilst it was there.
The prefer vallisneria over algae and poop tons. They will mow down all your plants if you are not careful. The size maximum size is way off, I have seen wild ones anywhere from 10-15cm (fist sized). Population is very easy to control as eggs are laid outside of water in clutches of (for bridgesii) 200-600 eggs at 2-3.5mm diameter per egg. All Pomacea bridgesii are apple snails, but not all apple snails are P.bridgesii. ;)
This is often refered to as the "Mystery Snail". Unlike the apple snail, which is a different species, P. bridgessii does not eat healthy plant material, and works very well in planted tanks to help eat uneaten food and some algae. Just be sure to have enough calcium in the water for their shells.
There are many species of apple snail, some that grow to be softball sized and some that WILL eat your live plants. Check out applesnail.net for more info. P. bridgessii/diffusa are often called "mystery snails," grow as large as a golfball over their 1-1.5 year lifespan, and will not eat live plants (they prefer softer materials to digest). Really active, fun to watch snails!