I've got three- the Eheim Auto Feeder, Nutramatic 2X Automatic Food Feeder, and the NutraFin Pro Auto Feeder.
As a student, I unfortunately have to rely on them when I go away for Christmas. I have used both 3 times, for periods ranging from 2-3 weeks, and had NO problems.
There are obvious problems associated with auto feeders and everyone says to stay away from them and get someone to feed your fish, but the reason why these are on the market is because sometimes you have no other way!
Test them a week or so in advance... put new batteries in, test the output level, and make sure they work properly.
The said, I really like the NutraFin Pro Auto Feeder. The amounts are rather consistent (I use pellet, I wouldn't recommend using flake in these) and it worked perfectly. The feeder fit the canopy on my 20g perfectly (it has a pre-made spot for this feeder), so that was nice.
The Eheim is excellent, well-made and has a little fan that runs during each feeding, to help air the food out (not sure how well this actually works!). The one problem is it doesn't mount on the tank with a bracket; it requires a horizontal surface. I have a glass cover on my 55g tanks so it worked, but it sketched me out. basically there are heavy plates on the one side, so it weighs it down good and lets the hopper hang over the opening to your tank - again, sketched me out!
I'd like to have the Eheim with a nice bracket, similar to the Nutramatic 2X. The 2X is pretty cheap, and you can tell when you handle it. The bracket is very secure though, and it worked perfectly on my 10g. Basically you use a screw and just tighten it to the glass on the side of your tank, so it perches on it securely. There is no programming either - the moment you stick the batteries in, it'll feed and do so every 12 hours onwards. Annoying to try and fiddle with proper feed amounts, because you gotta keep taking the batteries out and putting them in.
If you have a horizontal surface on your tank (i.e. a glass top) and are confident, the Eheim is excellent. The nice thing with the Eheim is you can program more feed times than the Pro, and it has a function that lets you feed the fish the same amount 1 minute after the initial feeding. That way the fish aren't just dumped with a lot of food, you cut it in half and hopefully the smaller guys can get some during the second feed.
Like I said, I've been lucky. I get a guy to clean my tanks once a week, but aside for that, I rely on these things every XMas. They'd be a good addition if you wanna feed small amounts multiple times a day...
