I have one and we use it about once a week, usually when we are not at home (out 2-year old and Roomba are not the best friends).
It pulls a decent amount of dust and hair from the carpet.
I once vacuumed with the upright and then let Roomba do the carpet again and at then end of the job the Roomba was empty, proof that the upright did a decent job. I plan to do the opposite sometimes, let Roomba do its thing and then come with a clean bag in the upright and see what I pick up after Roomba, but so far I haven't done the test.
Overall it's not bad. I would agree with olnacl, it is not a necessity, but for me it is a nice help since I don't have time to vacuum as often as I'd like and Roomba fills in the gaps quite nicely.
Also it goes under the couch and other places where I cannot reach without moving furniture, on the other hand misses corners and other tight spots where the upright also does not fit, but at least I can reach in there with the attachment hose.
I got my Roomba in June, the battery still holds a good charge, but I read that after a while you have to get a new battery pack, or at least rebuild the battery pack with Sub-C NiMH cells, and you can get higher capacity, 4500 mAh instead of the OEM 3000 mAh.
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http://www.roombareview.com/chat/