I have sat many hours in an Aeron (and am sitting in one right now), but am not sure why I don't hear more about the Herman Miller Sayl Chair. I find it vastly more comfortable.
I bought an Aeron a few years ago, but also considered the Sayl. I went to a Herman Miller dealer and tried all the chairs. A salesperson talked me through the different adjustments for each chair and explained why people preferred each of the chairs. When someone can sit in them side-by-side, they will pick the one that they like the most, which might be the Sayl. But many people only know the name Aeron.