I have an Aeron and a Mirra 2. The Mirra is both better looking, better constructed and more ergonomic. There are more adjustments, and the mechanisms are very intuitive and integrated into the design. Perhaps the new Aeron will bring it up to par.
I have both as well and the Mirra 2, in my mind, is the way to go. For me, the Aeron has the edge of the seat part where the back of my thighs rest and it starts getting uncomfortable after 15 mins. The Mirra doesn't have this issue. The Mirra back also hugs my shoulder blades a little better. The plastic back of my Mirra doesn't bother me vs. the Aeron mesh.
I would recommend the Mirra for anyone looking at Aeron. They're usually cheaper too. I got my Mirra on Amazon for around $600 which the Aeron was going for, I think at the time, around $900.
I always get Aerons used on CL. In the Bay Area, if you shop around for a few days, you can get one for $300. My last one came from a dentist office. I sold my OK-condition size C for $280 and picked up a better-condition size B (which is actually my size, at 6'0 and 220# with a 44" chest and 36" waist) for $300 later that day. It was the second one I called about; the first person didn't call me back fast enough.
Agree that the lumbar support is more comfortable but PF does have some benefits. I like that they redesigned the arms which support the seat above the part which connects to the top of the gas cylinder. Hopefully this will have less tendency over time to wobble.
Besides, the Mirra 2 is beautiful, while the Aeron is... debatable. This is mine: http://www.hermanmiller.com/content/dam/hermanmiller/page_as...