I've been a web developer for the last 10 years, so have had my fair share of fun & madness with all of them.
I won't use IE on anything... piece of junk. It may be a great browser for some, but as a web developer, it has caused more problems than all other browsers put together.
Opera is nice, I've tried it on several occasions, but there is something that just doesn't sit right with me... if they opened their plugin architecture so that 1passwd could support it, I'd give it another chance.
Firefox is great... I really like it. The add-on's are great, and I've been using it more since I started using Google Docs and Google Mail (so I can use Google Talk). The only downside is Firefox is so slow on Mac. It takes MUCH longer to load than Safari.
On the Mac I find myself returning to Safari nearly every time. Safari,for me, is FAST, STABLE, FAST, causes the LEAST amount of PROBLEMS, FAST and works properly with just about everything we throw at it. Did I say it was fast too?? Safari's roots are in the linux world too, so the stability doesn't surprise me.
As for the Windows Safari option.. I'm happy to see the browser there, although at the moment it's still early days. The initial load time is slower than Firefox, but render time is pretty much the same. I think there's a lot of promise for Safari, and when Google get's things working with Safari, I'd use it on PC & Mac. Until then, I'll stay with Firefox on PC.
Sorry for the ramblings!