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I bought a roomba about 5 years ago (only £250 back then). Have had to replace the battery (easy job, they're modular so it's simple to swap parts) but other than that it's still going strong.
Works well, picks up a lot. It does take a lot longer than hoovering yourself but given you press the button and bugger off that makes no difference to me. It obviously can't do stairs, tops of skirting boards, etc so you may need to go round with a brush/portable hoover every so often.
I do have all wooden floors though, not sure how it would go with carpets.
I've always assumed robot vacs would be shit, but I suppose if you buy a £500 one it should be decent. Maybe I could program it to follow my cat around.