Yep, big poopers for sure. Contrary to belief they are horrible tank cleaners, mainly due to first reason, by also because they don't actually clean anything at either an alarming rate or an effective rate. although, the poop isn't like that of a fish in most cases, it contains a micro organism that lives in its digestive track to help it break down foods. When in the water though it creates a hazy fog. If you've had a fish tank a few weeks prior to putting the snail in, you'll definently notice a diffirence.
One thing that is fairly unknown is that mystery snails actually -do not- do well in tropical climate waters. This brings them out of thier comfort zone (although still survivable of course) and often times makes them incredibly less active then normal. This is displayed by the snail often sitting in spots for a day, with little to no movement, and often near the top of the tank or floating.
Also, be sure to check for metals in your tap water, as these can quickly kill snails.