Opera is nice and all, but from last time I remember the actual interface looked way more intricate than Firefox, which to me simply boiled down to the fact that less is smoother. A desk full of items will bring forth a feeling of tiredness way earlier than a cleaned-out desk. I browse quite a lot at times, so I prefer an interface that's easy on the eyes. And like I said, last time I used Opera about a year and a half ago, it didn't feel that way to me.
I won't deny that technically, it's a great browser.