Any chair is good if you don't think about it at all while using it and don't notice odd aches and pains after you get up. If this is the holy grail for you - and maybe it is - at very least check into the return policy before you commit. I have a nice office chair in front of the machine I use most, a somewhat inferior office chair in front of another machine and an antique round piano stool in front of a third. All work fine for me, but maybe I don't have the health problems that you have and would notice a big difference if I did. I have to agree with those who point out that the back rest doesn't matter if you're not inclined to lean back while sewing.
My DH was surprised that I liked the piano stool, but I moved it up here after it finally dawned on me that the backrests on my sewing chairs were extraneous. Apparently my heinie is well enough padded that a solid wood seat is no problem either. I love that it swivels, as do the office chairs. As long as I'm not fighting to get up and down, I'm happy.