I've had the Herman Miller Mirra chair since December 2011 and could not be happier with it. Much better than the SteelCase Leap Chair it replaced and in my personal opinion it's more comfortable than the Aeron chair.
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)
After trying both, I went with the Aeron. But before that, I used a Leap for many years and I was quite happy with that one, too. You just have to try them and see what works for you.
