I've had one for 4.5 years, and about 2 months ago this problem got bad enough that it started to really bother me, and I looked into what was actually causing it. Turns out the foam is pretty worn out so sitting for a small amount of time compresses it completely. I bought a new chair (lightly used for cheap on craigslist) and I'm back to comfort. I think the is probably the result of any chair with a foam seat, but at the annual price it's hard to beat this chair for value and comfort.

I'm 92 kg and 188cm for comparison.