I just bought one of these, I've been testing it by having it hang off the side of the tank backwards (so that it is dropping off the outside of the tank and onto the shelf) and it seems to be working fine. It drops a few pellets into the tank every 12 hours.
http://www.hagen.com/usa/aquatic/product.c...=01107800020101That is the one I bought.
You can adjust how much food goes into the tank, it takes some trial and error to figure out how much is the right amount though, so I would recommend a few dry runs before you use it over your tank. You can force it to drop food by taking the batteries out for a few minutes and then putting them back in.
I have yet to use it in my actual tank as I haven't gone on vacation yet. But I'm going to be gone for four days over thanksgiving (yes I know they will be fine for four days without food, but I'd prefer they be fed), and it seems to be reliable.