Although not everyone likes them I do recommend shelling $$$ for an Herman Miller (or similar) chair.
The Mira seems to be great for the back. Me, I can't live without the "pellicle mesh" of the Aeron so the Mira is a no go for me. Hence I bought an Aeron with the adjustable posture fit (the new one, with the adjustable stuff that gently "pushes" the low-end of your back).
I realize HN is filled with negativity and a lot of people are going to criticize the Aeron (dot com symbol, not that ergonomic, bad for your health, bla bla bla) but honestly if you know how to adjust it it's a great chair. You hardly sweat in it and (when adjusted correctly), by pulling your arms behind your head the chair gently falls in "relax-mode".
The 12-year warranty is great too. I use that and a HHKB Pro 2 as my keyboard.