I have a Hermin Miller Aeron as my office chair, a Herman Miller Mira as my home office chair and I can sit on them for 12 hours with zero back issues (not that I would sit that long without moving) I had a Markus for 3 months. Whether you agree or not one gave me back pain, it wobbled after time, a wheel broke and after 2 months I had massive coccyx issues. It is therefore in my experience a bad seat if, like me you suffer from a weakness of the pelvis (it twists) which in turn exposes the coccyx it is not of a good enough quality to prevent that.
To my point, if you are fine then these things do not show themselves so I can understand how you think it's great but I don't feel the need to buyer justify either as I bought one and in my experience it was garbage.
Each to their own however and I agree go try a few OP but sitting in a seat for 15 minutes in a shop will show you nothing.
If you had back issues etc then you should have a specialist chair which is very adjustable and not something from Ikea, that's common sense. It does not however mean that everyone needs one, you have a bad back, amongst other issues, I do not. I'm far more likely to have a bad back after the cheap chair I'm having to use at work than the Markus.
You're comments however make it sounds as if the Markus caused your bad back, if so, please let us know how you got on with the legal case against them.
There are dozens of OCUK members who have Markus and gaming chairs and have used them without problem for years.