I've been using FF4 since the beta started, and I've been very happy with it (there's a thread on here if you want to dig it out).
Functionally superior to FF3.x and IE8 - I really miss it when I'm working on other PCs.
Once I've finished upgrading to Win7x64, all my PCs will be running it (32-bit, though, as Mozilla haven't got there yet with a 64-bit version).
SD