I've seen lots of Roomba knock offs, as answered earlier, the point is just to make money.
With regards to robot vacs in general, my Samsung Powerbot actually works pretty well. I have two cats and a dog, so it helps keep the carpets looking a bit cleaner than they otherwise would in between using the real vacuums. My only real issue with it is that the bin fills up and clogs quickly, I used to have a Neato that worked about as well but didn't have the clogging issue. I've owned many robots in the past, but before I bought the Powerbot I bought a Roomba and returned it. The Roomba never successfully completed a cleaning cycle in the week that I owned it, there was always something that it managed to get stuck on. The Powerbot is good about navigating around obstacles and not getting stuck, I think it has failed to complete a cleaning cycle like 3 times in the nearly two years that I've owned it.
I think that what trips most people up is that they lose sight of the purpose of these machines, they aren't intended to replace a regular vacuum cleaner yet. They are great for someone like me who just wants clean-ish carpets without vacuuming every single day. That being said, there definitely are some people who abuse them and completely rely on them to clean their floors. I have an ex whose family had two Kirby vacuums and an Electrolux, but they exclusively used their Roomba. They had a 1.5" band of black dog hair along all of their baseboards where the Roomba was unable to clean. It was also one of the dustiest houses I have been in, I realized their Roomba had a hole in the filter that was releasing dust every time it ran.
I would like to say something about what Mike said in an earlier post, "...just as a cordless vacuum will never clean as good as corded vacuums do, robot vacuums won't either."
I don't think it's true that cordless vacuums will never clean as well as corded machines. In fact, cordless vacuum cleaners have the potential to be more powerful than corded vacuum cleaners. A corded vacuum in the US is limited to 12 amps at 120 volts, while the power output of a battery can theoretically be even higher than that. I think it is unlikely that we will see cordless vacuums that draw that much electricity, but I do believe cordless and corded vacuum cleaners are getting closer and closer to matching each other in performance. Batteries can have a higher power output than a regular outlet does.