It DOES suck - in a good way...
There are a few posts on here that say that the Roomba does not have a vacuum function, or that it does not suck - this is inaccurate.
It's true that the majority of the cleaning is done by twin counter-rotating bars (one to brush the carpet and one rubber one to beat the carpet), but the beating of the carpet brings the dust to the surface (much like if you whack an old sofa or train seat), and then at the back of the Roomba is a vacuum - albeit a low power one - which sucks up this dust.
I have a Roomba 560 which cleans my flat daily, and leaves it spotless. There's no point in having one if you use it infrequently as it's not meant for deep cleans, but with it running every day my carpet looks great.