I've been using Firefox3 as my primary browser since a month before it's release. I had tried using the alphas and betas, but there was an essential feature missing on the Mac - keyboard shortcuts like cut and paste didn't work in dialog boxes. It was fixed in the first release candidates, and I've been using it ever since.
Just about the only thing I don't like about Firefox3 is the default theme. The default for the Windows version is actually very nice looking, but the Mac default is this hideous all-brushed aluminum thing. I've been cruising for replacement themes, and I picked up a Camino clone last night which I am very pleased with.
Other browsers I have installed: Safari, Sea Monkey, Camino, Firefox Minefield (custom built), Amaya, Flock, Opera, and Webkit (test build of Safari). They mostly get used for testing stuff.