Yes, i have one and love it! Got it when i bought my sitdown longarm last year. I was a little skeptical at first, but quite honestly i can quilt for hours and be comfortable and not have a backache when done.
And it really is helpful for your posture in that it helps to strengthen your “core” muscles. I went to physical therapy a few years ago to do that after a lower back injury - its a lot more fun to accomplish the same thing while quilting! I really noticed during quilting that i am not stiff or fatigued when i get up to do the usual bobbin change/take a break and walk around thing ya gotta do! That was a very pleasant surprise!
At first you really have to pay attention to how you get on and off, but thats only because its just different from what you are accustomed to when “mounting and dismounting” but i dont even think about it anymore. Mine adjusts in both height and pitch of the seat.
I havent noticed ever seeing one at a quilt show that you could try out, but that sure would be a good suggestion for vendors! When i ordered my new machine last year, i asked about the chair since i had only heard about it or seen it in photos. They offered to bring me one to try out when they delivered/setup my new machine - how could i turn that down? While the guys were setting my machine up i sat in the chair and rolled around, etc (i have office chair mats placed where i sew or quilt since my Happy Place is carpeted) and it was great! I told the guys “you’re not taking this one back with you” and i called in my credit card number to the shop to take care of the details! If i remember correctly, it cost me just under $200.00 including tax.
Anyway, im just one person but it gets a thumbs up from me! I have been using it for a little over a year.