[equalunique]

I too have a Herman Miller Mirra. It would be the perfect chair if only it had a headrest. Aftermarket headrests exist for the Aeron, but I could not find any for the Mirra.

Beyond the adjustability that comes with the Mirra, I like this chair because the seat is actually a flexible mesh, rather than a traditional soft cushion. This is best for me, because it allows me to sit longer. By comparison, traditional soft cushions eventually become not-so-soft, and this will lead to pain in my tailbone. Mirra helps me avoid that pain, so I can sit longer.

I have a IKEA MARKUS too. It is perhaps even more comfortable due to it's height and headrest, but because it lacks a mesh seat like the Mirra, it's not great for long periods of sitting. (IMO)