Can anyone tell me if these things (as in any "robotic" vaccum cleaner) works as well as the manual way?

well, it doesn't work well on carpets.
I don't know about bare floors

It will not beat your Hoover, but it works for what it's worth. It is good for what I purchased it, to keep the hardwood floors with area rugs clean on more frequent basis. It pick enough stuff from the floor to justify it's use. It gets only stuck in loose cables (thought on few occasions it managed to get out of it as well) and carpet tassels. It makes it's way between chairs as well, no need to put them up, just space them so the Roomba can get between them and legs of the table.

And it cleans under beds and any furniture with enough clearance to let it go under. And, for another 20-30 bucks you can buy a Scheduler Remote and set it up to start unattended at predetermined intervals.