I was looking at getting an Aeron. The advice I got from an ergonomist friend was that it was good, but not quite as good as the hype would have you believe. The primary criticism was that the mesh structure could only offer a limited amount of support, and that as its tension slackened over time that would only get worse. There's no doubting that it's a very nice looking chair, though, and of a good overall ergonomic standard.
In the end I went for a more traditional ergonomic chair, an RH Logic. I don't suffer from back problems, but sitting at a desk for as long as I do each day, I'd rather keep it that way! I took my ergonomist friend's advice and went and tried out the chairs on offer in a local specialist shop, and the RH Logic was by far the most comfortable for me. The stress on "for me" is because if you're serious about getting a chair for ergonomic reasons then you need to make sure it fits you as an individual, and that's doubly important given the amount of money you're likely to be spending!