I'm using an old Marquardt Mini Ergo(modded to ps/2-usb meanwhile) which is split, but fixed. Either I am so used to it by now, that I don't want to use anything else, but that is not how I remember it when I got it around the year 2000, or slightly before that. It felt like it was made for me!

But I'm not sitting like a robot in front of it. My office chair is set to sviveling back, which I often do. I'm even exchanging that chair for an inflatable big rubber ball to sit on, from time to time.