I have a MIRRA chair since 1 year and this chair is really awesome! But let me mention some points you should take into consideration:
1. Mirra is much cheaper in price than Aeron ("fully loaded" version of Aeron is really expensive!)
2. Mirra has one size ("Whether you're a sumo wrestler or horse-racing jockey, chances are you'll be comfortable!)
3. Mirra is the first real "green" office chair
4. Mirra has a lot of colour options (mix your own chair!)
5. Mirra is comfy
6.Mirra looks stylish (also for the home office!)
Here are some useful links:
http://www.pixelsurgeon.com/reviews/review.php?id=439
http://slate.msn.com/id/2085064/
http://www.fortune.com/fortune...18467,546679-9,00.html
"I've got a Mirra chair here at work. Very comfy, and it rolls fine on carpet! I've also used the Aeron chair too, and I'd say the difference between the two is minimal, in terms of comfort."