I have a couple of 400 series Roombas, they've served me well and I've rebuilt them more times than I can count. The real weak spot on these is the brush deck drive motor gearbox as well as the brush deck itself. The motor area gets full of lint and the motor overheats and melts the gearbox. Other times the gears strip out and iRobot has never offered just the gears or even the whole gearbox as a replacement part. The next issue is that the drive sockets that engage the beater bar and main brush are far too shallow and strip out easily, especially when cat hair gets wound around the end of the brush.
Other failures I've had, the mosfet that controls the brush deck motor burns out, this happens particularly when the motor wears and starts drawing too much current but it can happen on thick carpet as well. I finally got some protected mosfets I'll try the next time this happens. The wires to the front wheel that detect rotation break from the suspension action flexing them. Several times I've had IR LEDs in the drive wheel tachometers get weak, same happened once to one of the cliff sensor LEDs. Brush deck and vacuum motor bearing failure, clay based kitty litter is especially hard on these, I found suitable replacement motors from Jameco IIRC. Speakers have failed on both of mine, I replaced them with small paper cone speakers, they sound better too, not that it matters. Side brush gearbox gets hair wound around the shaft and eventually has to be taken apart for cleaning.
In a nutshell, I'd have a hard time recommending one to someone who was not technical because they are maintenance intensive, unless you have so much money that you can buy a new one when it breaks, in which case you should just hire a human maid. Personally I'd much rather tinker with robots than vacuum the house though, and the Roombas significantly reduce the frequency I have to do a proper manual vacuuming.