They do 'give' a bit on big bumps, but don't do a lot for all the smaller stuff and vibrations that gives you a sore arse. I don't think it would make a road bike more comfortable. But might help on a say a hybrid used on forest roads, canal towpaths or tracks.
They sag when you sit down on them which some find odd.
Mine was a USE model, which I initially bought when I did the old Mountain Mayhem 24hrs at Sandwell Park on a dry year when lots of the course was 'corrugated'. It did help reduce the punishment from peddling hard on this sort of terrain. But a full suspension Santa Cruz has proved an even better solution since, so the seat post has remained unused apart from a cyclo-cross day out on the HONC.