I've forced myself to use Opera exclusively for the past 2 or 3 weeks. It was quite pleasant in some respects, I really want to keep using it, but also annoying in others (running 10.53 Linux beta on Ubuntu 10.4 and 9.10). I hope it gets better with the new version. There are a few showstoppers.
It's a very geeky browser. Tons of options, menus, sub-menus, etc. etc.. Which I liked, because I'm of a geeky persuasion (always preferred KDE for example, since it's so much more option rich than GNOME, or used to be before KDE4...). I quite like the Speed Dial thing, too. And it does feel quite faster than FF on my 8-year old home computer.
The problems? First, why the hell do the imported (from FF) bookmarks lose their original order and become alphabetical? That is quite stupid. Of course, after looking for the bookmarks two or three times you get used to the new position, but the old places, after years, are quite automatic, and should not be messed with. I looked for any option that tells how to order the bookmarks, but found none. Also, the bookmark layout is terrible to navigate, for some reason.
Second, it mangles several pages. It seems to have a problem with some CSS, I suspect. Those pages display fine on FF. E.g. some Yahoo News pages get all the content squished in a narrow vertical strip of some 50 px or so. Others, like the local Weather.com page gets elements all over the place, out of alignment. Most other pages look fine.
Third, the auto-completion in form text fields is junk compared to FF. It only remembers the few default ones you set explicitly in some configuration tab. But I have several email addresses, for example. I don't want it to always complete with that one only.
Fourth, the user/password management is flaky. Sometimes it works, sometimes it doesn't. Depends on the site, it seems. I couldn't figure it out. But for El Reg for example I am sure I asked for it to remember the details, but it never fills them.
Fifth, it does not recognize the Java plugin on pages, although it sees it is installed (in the content details configuration tab).
I don't know if there is a "bug" report system for Opera where I could report these things. I never bothered to look because I am not submitting anything to something that is not FOSS.