There was one fitted to my Ridgeback, I don't really think it makes much difference. There was one on my Sirrus too but when I replaced that with a Carbon one (with zerts) again I didn't notice the difference. What I do know is my old frame was too large for me and after fitting a Suspension Seat Post I started to develop a sore knee; replaced the post, it went away. Having said that I probably didn't pre tension it correctly.
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.
As far as I'm aware the best one on the market is Cane Creek's Thudbuster. It uses a linkage design rather than telescopic. Its been around a few years and does make a real difference. However its quite a bit of money, and of course you still get the vibration through the bars and pedals.
It gets even more interesting,i have never seen anything like that seat post before,incidentaly i was out on my bike again and the ache didnt show itself anywhere near as bad as last time,but there is a lot of food for thought,thanks for the ideas guys
I've got one (not a thudbuster), which is quite good, you can adjust the tension in it, its quite heavy though and I'm beginning to think that it might not make an awful lot of difference without it, it was just on my bike when I restarted cycling a few years ago so I've kept it. I have heard good things about the thudbuster tho - but its pricey.
A useless item.
There was a Post Moderne, fitted on my hybrid when I bought it. I only cycle onthe road and never thought it did any good! After some usage it became loose, allowing the seat to rotate through a small arc. This could be fixed by tightening an Allen screw, but quickly became loose again and could not be tightened further.