I have a Haworth Zody chair at home (Craigslist in Grand Rapids, MI, office furniture central) and another Haworth chair at work, and I assure you, it makes a difference. I felt it'd be out of place to drop name brands in the "how much does comfort in the office affect productivity" thread a couple days ago (http://news.ycombinator.com/item?id=234445), but oh man. (NB: I am tall.)

The Steelcase Leap chairs are really nice too, and I have no doubt that the Herman Miller chairs are great. Absolutely try any of the above and see how they fit first, though -- there's another Steelcase one that just doesn't work for me, at all. They're all expensive, too (craigslist nonwithstanding...), though one that fits you perfectly is well worth it in the long run, and they all seem to be built to last. As expensive workspace stuff goes, they'll certainly depreciate more slowly than a huge, expensive LCD monitor.

Just like keyboards -- some people love the split keyboards (including me), some people can't stand them but love the clacky ones, but the important thing is to have one that actually fits you if you're going to be using it several hours every day.

That said, I bet Jeff Atwood gets an awesome Amazon commission if anybody buys one of those through his blog. Yikes. :)