I've had a cheap-o that I got at Christmas Tree Shops that worked fairly well. Basically, if the stuff was already cold, it would do a great job at keeping it cold. But if you are expecting to take something from room temperature to nearly freezing you may be asking for too much (from the cheap-o brands).
In my opinion, they are worth the cost. I think I picked a small one for around 30 bucks. Coolers without any motors can cost as much.