Opera's quite nice in some ways, but the UI pisses me off more than it pleases me. The only thing I prefer is how the address bar and navigation buttons are inside each tab, rather than above the tabs. Otherwise, there's really not a lot that Opera is useful for which Firefox isn't, especially with extensions. That said, Opera Mini or whatever it's called, is really cool.
Making it available for free (as in beer) was a step in the right direction, but it's a shame it's not open source and free as in speech.