I've been playing for a few hours with Opera 9 tonight. Nice browser. But not sure I'll switch. IE 7 and Firefox serve my needs well and don't leave a lot of room for a new browser.
Will Langford, though, did a nice writeup of Firefox vs. Opera. I don't share his concern with RAM usage, my main thing is speed of opening a new window and completely rendering a Web site. Firefox is fastest here on my computer (which is running beta 2 of Windows Vista). I love the Bittorrent integration, I wish everyone would build that into their browsers. That's going to be how we're going to get HD video from my camcorder onto your screen, so I care about that a lot.
One other thing I care about: I'm going to be using both Mac and Windows OS's in my new job and I want a browser that's as close to the same on both OS's as possible. That leaves me with Firefox.
On the other hand, Opera has a nice browser for cell phones that I'm using more and more. So, it makes the choice hard.
Anyway, which browser do you like best, and why?