I saw a review for one in WSJ just the other day (don't have a link, sorry). They made a good analogy which I will have to paraphrase from memory.

The robot vacuum cleaner technology is a lot like a dancing dog, in that you are spending so much time being amazed that the dog can dance at all that you fail to notice what a horrible job it does of it.

Never used one personally but that quote stuck out in my mind.