I have Malaysian trumpet snails and have kept them with great success. They help break down dead and dying plant bits and do a wonderful job at burrowing into sand and/or soil; you may never see them during the day as they tend to be nocturnal and spend the daylight hours rummaging around under the surface of your aquarium substrate (though some may reside above or within plant clumps).
I started with six. I now have dozens. If you monitor the amount of food you put into the aquarium and are diligent with siphoning (to a degree, that is) you shouldn't have too much of a population exlosion though in soil based tanks this apparently doesn't have as much of a negative impact as in a non-soil based tank (probably due to the plants growth sucking up all ammonia produced by the snails, they don't create as much as other snails mind you).
I'd recommend MTS to anyone with a planted tank who uses a substrate that can compact; not only do the snails stop it going toxic but they are a joy to watch. Mine sometimes climb the side of the tank and suddenly let go, it's also interesting to watch the tiny tiny babies grow into thriving adults. I love 'em!
... I do wish I didn't have so many though.
I've also had no issues whatsoever with them co-existing with ramshorn snails.