So, I've got a bad back (lateralised discopathy L1-L2 L2-L3 L4-L5), and after spending a week on my back with twice-daily valium injections, I'd decided to do something about my seating arrangements. I was looking at a "posture chair", but they are hideously expensive, and so I was going to make one.
And then my physio gave me a bunch of exercises for the back, all of which revolve around one of those big gym balls. So I decided to do my exercises whilst programming, and have found that one of those big gym balls, at 29 euros, is more comfortable than any chair, ergo or not, I've ever used.
the only downside is a tendency to get a sweaty arse.