I've never left Firefox (if only because Chrome on my OS has been made available only a short while ago), however I must say I'm pleasantly surprised by the performances Firefox 4 beta 4 for Linux 64-bit (the Mozilla build) brought over my distro's default 3.6.8 build.
The interesting thing though, is that on Linux, the 3.6.8 "software rendering" goes as fast as 4.0's hardware-accelerated rendering on many IE9 demoes (psychedelic browsing, for example, goes well above 1000 rotations on Firefox 3.6.8 for Linux - but it actually depends on hardware drivers, on Intel crap I get more than 1000 on a 600x600 window, if I go over 700 it falls under 1).
Panorama is mightily interesting, but I'm not so sure about Sync - although if it really is secure, I'll probably use it heavily: between work, home, netbook and various VMs, it sure would be useful.