Well, I do lot's of sport and can sit comfortable on hard ground meditating for quite some time. I still enjoy a good chair way more than something "normal" for any longer computer sessions.

If a chair is too comfortable, I just fall asleep in it. It doesn't make me work better.

A chair isn't any answer to poor posture. The answer is exercising your core muscles, being aware of your posture, and constantly correcting it.