I like Opera—I remember using it way before any Mozilla products—but I just don’t find myself using it as often anymore. I agree, it has so many things out of the box compared to its Mozilla counterparts (perhaps Flock included!), like what Ferdz mentioned as well as widgets.
Oh, and the latest version of the browser uses Ctrl+T for opening new tabs now (and Ctrl+N for new windows), unlike before. :)