IMO, the Mirra chair is horrible. I’ve had the pleasure of having that chair as my work char on two different jobs, and both exhibited the same problem: the plastic back hurts after a while. I can’t stand it. Also, I like to sit on one of my legs sometimes, and not a single one of the chairs shown permits that (without breaking your leg, that is). My $350 Staples executive high-back §leather chair is the best chair I’ve ever used, and, oddly enough, it too has lasted over ten years. I’ve played games while sitting in it for 12 or more hours straight with nary a pain, and I’m no spring chicken ('course, I’m not that old, either).
The only thing worse than a chair that doesn’t work for your body or sitting style, is a desk that is too high or too low. I sat at a desk that was too high on a job (being a contractor, you get what you get) and it caused a great deal of back pain for weeks until my back gave up and went with the flow. Getting used to a regular-height desk induced the pain again, but, again, my back went with the flow eventually.