I know for a fact that mine are doing their jobs...and quite well. I ordered 10 from this website and put them in the tank this afternoon. I *had* an algae problem on the glass, mainly due to the fact that it is a new tank that just finished the cycle a week and a half ago. Now I can see a trail leading from the sand up the glass to where each snail is...they just cut right through the algae.
My ceriths are sifting through the sand as I can already see new "mounds" on the sandbed. My hermits are taking care of what was on the rock with a couple of the astreas.
All in all, I am very pleased with my swf.com order. Trust me, your snails are not going without food. They are keeping the glass just as clean as you want the rocks to be...and they will be.