* It doesn't force you into a straight-back posture, but it makes it as easy as slouching. If you stop concentrating you don't fall into a poor posture.
* There's really only one way to sit on the chair, with your sit-bones taking most of the pressure. In a normal office chair I can switch between a set of different slouches to move the pressure between points, but not with this chair. I bought a height adjustable desk to compensate and switch between sitting and standing several times a day.
* It takes a fair amount of strength. I don't remember if it took me time to build muscle initially, and on a day to day basis I don't notice any tiredness or anything, but when I'm sick it is _really_ hard to sit on this chair - I feel like I'm going to pass out (maybe not to that extent, but it's quite noticable).
* I bought the first one for $800, but AFAICT they've come down to $300ish this year maybe due to competition. They're sturdy and really nicely built, but in the end it's just a stool with some curves.
* May or not be related, but I found my back getting really stiff because it is literally straight all day - sleeping, standing up, sitting on the Salli chair, etc, which worries me a bit.
* You sit at a much higher level than with normal chairs, so you'll need a higher desk to accommodate.