I've been using mine now for over two years and love it. I had a herniated disc in my lower back, and this chair helps my posture and to avoid the pain I felt in a regular chair. My pain is gone now, but I continue to use it daily at my computer desk. The seat was originally a bit too firm for my taste at first, but has softened up nicely over time.
Got the chair put together, then the problem started. Went to adjust the hight of the chair and the adjust was locked up. Had to disassemble the chair and put the adjuster in a vice so I could break the ends free. After about an hour, a decent amount of PB Blaster lube and some good old brute force I got it working properly. Unfortunately, by doing this I round off the grip part of the adjustment device. Put the chair back together and it seems alright. However, time will tell though.
The chair arrived less than 2 days after I ordered it and I put it together in less than 30 minutes. Both the seat and knee pads are very comfortable!