I went away for 8 days once and placed a ceramic topfin, fish shaped timed feeder in their for my one betta in a 2.5 gallon filtered tank. It did well. I have heard of some of these time release ceramic jobbies messing up water balances, but I had no such experience. When I came back there was a small pile of debri on the bottom left over from the feeder. I simply siphoned out the debri and did a 50% water change and all was fine. I know that my fish ate from it as well. The second I put it in the tank he sat and watched it untill it realeased something.. which he ate without question.
its up to you. If your tank is filtered with low current, fake plants and only one fish a feeder should be fine. Otherwise... I wouldnt risk it, hell be fine for 4-5 days.