There are some good ones out there and there are some bad ones out there...so do some research (besides this post) before you buy. I bought one from petco (can't think of the brand) but it's basically a tumbler that rotates 1 time every 12 hours. Found the problem with this model was that there's no way to change the timing, and it's hard to get a regular amount dispensed.
I would never use one in place of regular manual feedings. Even the expensive/nice ones can have problems if they get moisture inside of the holding area and the food starts to clump. Most of the also aren't very good for feeding anything besides flake food. My opinion is don't use one unless you have to. If you're going to be gone a few days (3-4), just let the fish fast a little. If you're going to be gone longer, give a friend the key and let them do the feedings.