Bought this for my son for Xmas but was over when it was on and not that noisy... Son has 3 cats who seem to ignore it.... Much quieter than a vacuum.... Works really good on rugs and also on kitchen floors.
According to the iRobot website, the specs for the 650 are:
Robot Dimensions: 13.39 inches in diameter, 3.62 inches in height
Unfortunately, I don't think it will fit under your radiators. For what it's worth, it won't fit under my couches either, which could stand a good cleaning!
I have (brace yourself) SIX (yes, six) indoor cats. Some long hair, some medium and some short haired as well. The unit does an impressive job of getting up the hair. I use the Roomba daily. After use the floors look great, I don't see any signs of cat hair. Sometimes I have to do a spot cleaning in a few areas that are popular hangouts, just to get all of the hair. Additionally, it does a great job of picking up food that falls on the floor and (my apologies for being a bit gross), dried up barf. Also picks up the (clean) litter that gets tracked around when the cats exit the box.
I have got a used one. Though I have got Neato -21 , it is a similar robotic system. It works. I think Amazon sells used ones because some of the people get those robots and decide to return then.Nothing wrong with the robots, just as it can not be called as new one anymore. So, it is here on Amazon, as "used, good" condition.
I also was hesitating, because it expensive but this Christmas i finally gave it a try. I tell you i think now of it as one of the best investments made. SOmetime it worth paying for those new technology things, because it does makes cleaning life :D much easier and saves your time for other things to enjoy. Why hire a maid and face possible future issues with the person, when for the same ( or less when you spread it over the year time period) money you get a robot who you can "trust" and not worry about some stranger in your house.
Good luck in finding "your robot" :D)