I've played with Opera a lot too over the years, I've never once said it was a bad browser, in fact I say it is better then Firefox in some ways, memory usage of course being the biggest one. I also absolutely love Opera's 'Notes' feature too, where you can easily take notes and access them from the right click context menu. Theres plenty of auto fill extensions for Firefox, and I've tried almost all of them and still can't find one that truly clones this feature of Opera, they just don't work in the same way or as well.
But, I too, always wind up switching back to Firefox after a few days, simply because, it just works. Just way too many problems rendering certain pages in Opera, when the same pages just work in Firefox.
Another reason too, maybe a small one but still an important one to some users, is Firefox is open source, which means, people are free to modify it and make optimized builds. I use a Firefox build optimized for SSE2 and its a little bit faster then the standard ones. This is not possible with a closed source browser like Opera.
