I guess I'll have to be the guy that stands up for Ilyanassa obsoleta snails. I can only speak of my personal experience with these snails. I bought 200 of these guys off eBay around 8 months ago and paid around $20 including shipping. These snails are unmatched in my opinion on their ability to turn the sandbed as they are constantly burrowing in the substrate. Out of the 200 I purchased, I would guess I still have around 150 of them 8 months later. So Tri, if you only have 20 left out of your 200 you recently purchased, then there is something else wrong there. I look at these guys as a cheap live food for the hermits and crabs in my tank. While they're waiting to get eaten or give up their shell to a hermit, they do a fantastic job of sifting the sand. I feel like I've more than gotten my $20 worth of use out of these snails and I've still got most of them still on the job. As long as you have a decent cleanup crew to take care of the ones that succumb, then I give these guys a big thumbs up.
All IMHO.