The Roomba absolutely does have suction - we actually had the vacuum part break a couple of months after purchasing, so we got to compare suction power vs sweeping power. It still ran, and all the sweeping parts were working, but the difference in the amount of dust/dog hair was incredible. The sweeper collected maybe 1/10th of what the sweeper+vacuum are able to pick up.
They replaced the part with no questions asked under warranty, and we've been happy ever since. It is such a lifesaver when you have a shedding dog in the house!