Problem with most chairs (including Aeron) is the bowl-shaped seat pan. It feels comfy / cradling when you first sit down, and that’s why people buy them. But as the minutes and hours go by, your pelvis is turning inward for lack of central support. Eventually you get back problems. Google Esther Gokhale for full explanation. An upwardly curved or at least flat seat pan (think old fashioned upholstered chairs or solid wood chairs) are more human designs.