We've had our iRobot Roomba robot vacuum cleaner for over a year now, so I thought that I should post an update. We're using the iRobot Roomba 531.
We still think it's brilliant and use it virtually every day. One day it goes round downstairs and the following day it does the upstairs. So our house is fully vacuumed every two days.
Battery life still seems good after a year of use. Maintenance is easy (it's good to regularly do a little 'service' to remove hair and fluff from the mechanism). Things that come apart for cleaning only go back in the right way round, which makes it simple to do. Apart from changing the filter we've not had to replace any components in a year of use.
Occasionally it gets snagged on a cable or tangled by a rogue sock left lying around, but it stops before any harm is done.
So if you can afford one, why would you buy a vacuum cleaner that has to be pushed round? The price of the Roomba is pretty similar to top-end manual vacuum cleaners anyway.
We have bought a Dustbuster to do the stairs, which is something that the Roomba cannot do though.
We've just found a supplier in the US who sell HEPA filters for our model of Roomba, so we're going to try those out next.
... and I never did get round to hacking it, partially because it's become so important to us, we don't want to risk breaking it. So the wife says that if I really want to hack a Roomba then I'll have to buy another one :-)