OK, I have two ergonomic desk chairs on completely different sides of the scale, both are mentioned in the thread.
Herman Miller Aeron: Fantastic chair with a few faults. Great if you're a bit warm blooded, and tend to sweat in most chairs. The mesh really allows your body to breathe. Can sit it in for hours. If you're shorter, like me, don't need to get a new center cylinder, as the Aeron has a pretty impressive height adjustment range. Not perfect though: no headrest. Doesn't bother when I'm actively typing, but when I want to sit back and watch a youtube video, or read a long article, I miss it. The mesh can snag on break on things. Not easily, but I've had to replace both my seat pan and back. But, its a fully modular chair, the Camaro or Jeep of chairs so to speak. You can build one from scratch with the parts catalog without actually owning any part of the chair.
Humanscale Freedom: Also a fantastic chair, but instead of having a myriad of adjustments, it has 6. Headrest height. Back height. Chair height. Armrest height. Armrest angle. And seatpan location (slides forwards/backwards). The rest of the moving parts adjust completely by themselves, depending on where your body puts pressure on the chair. I don't like the leaning mechanism as much compared to the Aeron, as the entire seat in the Aeron tilts, whereas the Freedom's back is the only part that tilts. But the auto-adjusting headrest is fantastic. Hell, the auto-adjust everything works really well on this chair. However, I had to hammer out the main cylinder to replace it with a shorter one. Which scraped my hardwood floors (they're not entirely even, sigh), so I had to replace the casters after that. Which meant taking apart the ENTIRE chair. PITA. Also, it uses circlips which I think are the most retarded mechanism useable to hold something together. EDIT - had to take apart almost the entire chair for the main cylinder, not the casters
I don't think you can go wrong with either chair, I like them both. I use an Aeron at work, so I tend to use the Freedom at home most of the time. I do advise getting the gel seatpan in the Humanscale if you go that route. I didn't and I regret it. I'll eventually replace it with the gel, but I have one of the spendier leather Freedoms which jacks the price up noticeably, so not exactly high on my list.
Both of my chairs are over 5 years old (the Aeron is about 9), so they're long lasting as well.