I've looked at some of the newer models, but haven't tried them. I vaguely remembering someone (Herman Miller maybe?) saying that headrests were a bad idea? No recollection of Why, tho I agree sure seems like it might help for the heavy headed amongst us.
Also agree on foam/solid padding - the mesh/suspension type are SO much better for long stays, and as you say, foam pads only seem comfy for the first hour.
One downside I had with Aeron (really all five wheeled chairs) is that there's ALWAYS one leg that's in your way, esp if you're in a rush to get on to something.
Upside is that I thought I might need some new parts (after 20 years!) and found there are a zillion parts options out there, including HM of course - but mostly ebay choices all over the place. Spares are no issue.
Also, that 'butt committee' did NOT bring in examples from places like Ikea or other retail furniture makers - only those on contract - so there may well have been other equally good chairs out there (esp now, maybe not so much in 2000s when Aeron was new and 'scarey' - and originally rejected by senior management as being 'too weird' and not 'officious' enough, but within a year they ALL had them!!
Yes, seems a bit sillly to obsess over chairs, but disk surgery makes me sensitive to issues - esp wrt to hrs long useage.
Happy sitting!