I have wisely changed my post since it's first draft, as it was originally unnecessarily ranty. (It's been a hard week.)
I've just got a new office chair. I'm 5 foot 6, and if I sit with my bottom all the way back, there's 4 clear inches between me and the back rest (which is non-adjustable). This chair is clearly made for people with very long femurs. I can sit with my back against the back support, but only if I roll my spine into a lovely "j" shape. And then I kind of gently bounce, because it's got a springy back support. (I look like I'm in a baby bouncer.) What is it with office chairs with springy back supports-that-don't-support.