I was going to get Aerons for our office cos every thread on here has that as the default answer but thankfully I did a bit of extra Googling and virtually every other source I found on the interweb recommends the Steelcase Leap instead – especially if you have any kind of back problem.
2nd hand V1s on eBay are very affordable – paid a bit more to get V2s but they were still way cheaper than new. Really pleased with them. They are very adjustable and making the adjustments is extremely quick if you want to change your position. Also the seat stays parallel to the ground when you recline which I like (unlike most other chairs including the Aeron AFAIK)
Steelcase in our trendy new place (think W1A). Lumbar support is rubbish – a simple slide up and down thing which just keeps sliding down.
Sure you didn’t get fobbed off with cheap Chinese copies? 🙂 Our 2nd hand ones are all fine!