I've got the Windows version installed, and I'll admit that I'm not gobsmacked at the moment. The build feels fast, but not noticeably faster than the nightly builds I've used up until this point. Benchmark scores are certainly good -- with the Firefox 4 preview hot on the heels of Chrome 7.0.517.0 on my system (a quick Sunspider run was nearly neck-and-neck between the two browsers).
Even on the Fluid Dynamics sim mentioned in the post from Mozilla's Dave Mandelin ran at a higher FPS in Chrome.
Maybe my system is having some issues, though -- so let us know! Grab the Firefox 4 JägerMonkey preview build for your platform and tell us how it compares to other browsers!