I have one of the battery-operated ones. Twice a day it makes a full rotation and dumps in food. I also have it on the lowest setting. I use it when I'm gone for more than 3 or 4 days. As someone on here said, fish can go up to a week without feeding, but I'm away often...
As for the kind you throw in the tank (3,5,7,14-day feeders that dissolve), I have never liked them. They're messy, and if I'm away, I really don't know how long they last or how fast they get eaten up.