Ever since I first found Opera (version 5 or 6) I've thought that it was the best browser around, even better than Firefox. In fact, I've never thought it was fair that Firefox got so much credit for things like tabbed browsing and mouse gestures (through extensions) that have already been in Opera for a long time. There were several things about Opera 6 and 7 that I didn't really like, but every complaint I have had about Opera seems to have been fixed in this version.
Before I found Opera, I used IE because I didn't know any better (and it wasn't nearly as bad then as it is now). Then I switched to Opera and absolutely loved it. I switched back to IE mainly because there were a few pages that I visited regularly that wouldn't work properly in Opera, and I didn't want to mess with switching between browsers. When Firefox 1.0 was released, I gladly accepted it as my main browser and even removed the shortcut to IE from my desktop, but it always irked me a little that FF was getting all the credit for "new" features that I first saw in Opera a few years before. Now, I'm switching back to Opera. The only thing that FF has over Opera IMHO is its extensions.