I'm 27 and used one at home for a long time. I really loved it and the only reason I stopped was that I got a hole in it. In general, it's not as comfortable as an Aeron, but it has a pretty high comfort to cost ratio.

That being said, since it was at home I was never on it as long as I would have been at work.

I'm 36, and have used them at home and at work for a couple years now. I gave away my old office chair -- not that it was a particularly nice one, but I don't need it any more.

I am 34, have used one at work for about one year, and love it.