One of my biggest complaints with Opera is that it's turning into the emacs of browsers. It wants to be everything: your FTP client, your IRC client, your browser, etc. I want my browser to just be my browser, with maybe a weather forecast and a custom skin at the most. I'll use a dedicated FTP client, IRC client, etc, thank you very much.
That was pretty annoying when I first used Opera; but you can disable support for anything you might be using an external application for. Opera's configuration is very flexible. It's just hard to figure out *how* something specific is done. The configuration never seemed very intuitive to me. It took me at least a year to really get comfortable enough to know what I was doing. (that is, with most any general configuration setting) Once you break it in, though, it's a great browser. Really, it's not Opera's basic feature set that I like, so much as its overall flexibility. I can make Opera do what I need Opera to do; and I can do it with more relative ease than I can with FF.
I am gonna agree somewhat with Zera here. The whole "let's do everything ever for you" was annoying when I started using Opera ( back in the 6.x days! ) but as I soon found out, I can install it as a portable app, or install it on some random part of some networked drive, and have all of my programs in one. So it was handy then.
Though on OS X it's a pain because there are still a fair amount of GUI bugs to keep me from using it. Not that I would because the rendering is slow anyway. But that is a different story.
And I was still hoping FX4 would have an update in GUI as well as back-end for OS X, but I guess that is what Camino is really for. Not that Camino has been updated too recently. The fact that FX doesn't look like a standard aqua app, is really irksome.