My experience is with the "Aquabot" line of robotic pool cleaners. The function of the product is great, it reduces the need to vacuum your pool and takes a lot of stress off of your filter system (and is also a lot easier to clean than your filter). The downside is bad though, the units on average are over $800, parts and service are very expensive, and in my experience they rarely last more than 2-3 years without needing major repairs.

Right now my current Aquabot is totally dead and I doubt I will be shelling out the money to temporally repair it or to buy a new one.