I just spent a large amount of time shopping for a new office chair. I had the fortunate experience of my local supplier letting me demo 3 popular Herman Miller chairs in my home for a few weeks. Long story short, TRY out these chairs if you can. They are not inexpensive and are a significant choice, you want to make sure you get the best fit for you.
Initially I tried out the Aeron and Mirra at the store. I really felt the Mirra was going to be the winner, but they insisted I try the chairs in my home. I'm glad they did! Turns out the Mirra is a problem for some people, as was the case for me. If you're one of the unfortunate few, your legs will go asleep when sitting in all but the forward tilt position. Sad, because I really liked the chair otherwise. I'm guessing a lot of people don't have this problem.
In the end, I bought the Embody chair, which is the newest model by Herman Miller, and I really like it. It doesn't have the hard lip on the front of the seat like the Aeron, and feels great when sitting in a slightly reclined position. The fabric is thicker than the mesh used by the Aeron and the seat of the Mirra, which I notice more now that it is summer and my office is warmer, but it's not a big deal to me.
Anyhow, I highly recommend trying chairs out and possibly an in-home demo if you can manage. Sitting in a chair for 15 mins at a store can give you a rough idea, but the true test is using it at your home as you normally would.