Opera is its own flavor of browser - the GUI is so customizable it's almost scary, and the usability enhancements can make it seem a little odd at first glance. But even though I have a rather heavily "loaded" version of FireFox, I still gravitate to Opera all the time. Just the native RSS integration keeps me smiling.
Funny, because I find exactly the opposite to b e true. Opera was one the very first, if not THE first, browser to use tabs ... and I would much rather have this implementation than any other I've seen. Even with extra Greasemonkey help, I still can't get Firefox to be as comfortable for me.
One small example - close a tab in Opera and the last USED tab is visible. Close a tab in Firefox and it goes down the line in order of when the tabs were opened, not when you last used them. A small usability touch that tends to save time.