First, find your Roomba.

Today it had gotten through the slightly-ajar lounge door, cleaned the exposed carpet and then wedged itself trying to climb over/eat the bellows.

Overall: it works. The house is cleaner and I can walk around barefoot without that ‘gritty’ feeling underfoot. My bet is that if it does 80% of the vacuuming repeatedly each night – a-la cronjob – then the house is cleaner overall, the 20% will tail off over time (since the Roomba will get most of the dust before it has a chance to move and collect in Roomba-resistant places) and the remaining work is is doable in hour or so.