It is actually very good, by virtue of running every day (unless you change its schedule to something else). So even though its suction doesn't compare well to a real big canister/bag vacuum, it makes up for that in sheer level of activity.
It's not perfect. I have a single-floor apartment with no door thresholds, yet it keeps getting stuck. I keep cables and so on neatly organized and hidden, but sometimes I've left a temporary charger cord or something on the floor; this kills the Roomba. You also have to careful with plants -- it ate a whole branch of a plant once that was hanging low near the floor. It has issues with rugs, especially with black areas, but also just getting onto the rug. It also loves to close the door on a room so that it can't get out; I had to buy a couple of door wedges to prevent that, but I don't always remember to use them. Pretty much 50% of the time I come home to find the Roomba dead in a corner.
Its dust compartment is quite small and must be emptied at least every 2-3 days. You also have to take out the little spindle things and remove hair which collects around the axles. Fortunately, the Roomba is really modular in design; you can take it apart completely, and there are replacement parts for absolutely everything. It is a rare product where the designers seem to have rejected the trend of planned obsolescence; I haven't really owned it long enough to be qualified to judge, or taken it apart, but everything I have seen indicates that it was carefully engineered to be repairable and last a long time. (It's probably hackable, too.)
Despite its weaknessss, I am quite happy with it. It keeps my apartment clean and removes the mental burden of thinking about dust. A couple of times a month I take my Dyson to clear up some corners that the Roomba can't get to where dust bunnies pile up. I have a friend who combines it with the cleaning model, but I can't deal with two gadgets that leave their depleted, cable-choked corpses around.