I bought one of the first Romba back in I guess 2006 or 2007. They're pretty awesome. You think you've done a good job of vacuuming until you see how much crap the Roomba picks up after! And possibly the other way around too .. they're somewhat complementary devices.
Navigationally they're very stupid. It's mostly a drunk walk around your house, just with a few different behavioural patterns that it follows for a more or less random time (a few seconds up to maybe 30 seconds) each e.g. "follow walls on the right side", "make 80 degree (or 100 degree) left turns when you hit something", "make right turns when you hit something","make a nearly 180 degree turn displaced by one Roomba-width when you hit something after a long (~5m?) straight run without hitting anything". It ends up over the course of an hour or so being pretty effective at discovering every room and covering every spot in at least a 50 - 80 m^2 level of a house or apartment.
Sadly, that machine is back in NZ and I've been cleaning manually in Moscow. Sadly the air here is very dusty and you need to clean hard floors every day, or two at the most, to avoid a gritty feeling underfoot and black soles of your feet when you go to put your shoes and socks on.
I guess this thread inspired me to notice a Roomba in the window of an icover.ru store as I was walking around near (but not tooo near) the protests yesterday and I picked up a Romba 681. The 616 was a bit cheaper (19900 rubles, AU$450, NZ$500) but for an extra AU$135/NZ$145 the 861 comes with a higher capacity LiOn battery instead of NiMH, and a few included accessories such as a virtual wall and spare filter and side-brush. Other than the battery those things are pretty cheap as spare parts anyway.
The thing is overall very much the same as my original model, but you can see small improvements everywhere. The most immediately noticeable difference is it now somehow slows down a lot about 10 cm before crashing into walls and furniture. The original just crashed into things at full speed. The new one gives a very gentle almost inaudible nudge. I don't know what sensor they're using for that. I'm guessing ultrasonic, but I haven't spotted a transducer. It's not sensitive enough to notice thin chair legs or the ends of panels on flat pack desks, and still hits those at full speed.
There's now a 900 series of Roombas for about twice the price with WIFI and remote and smartphone apps and apparently real mapping of your house using a camera and proper planning of efficient cleaning patterns. Meh. The cheap one works fine.