I've been using a snail snuffer system on my A-class during one season. I've used it during roughly 30 sailing days. I logged an average of 30 Nm each day and I hoisted the spi roughly 5-10 times each trip.
I've heard rumours about wear and tear on the spi. The only fact about wear and tear is from the Tornado homepage that was refered to earlier in this thread. My own experience on wear and tear is that I had no problem at all. As long as the spi doesn't get squeezed against the inside of the snail this system is much kinder to the spi than a normal snuffer.
If you have an uni rigged boat the snail is a good choice, you an mount the snail close to the mast which gives you low drag and good weight distribution. If you have a jib I would recommend a standard snuffer, bag or tube.
You can check this homepage for more details on the snail.
http://www.webbo.dyndns.org/a-cat_sailing/spi//hakan