U_D said pretty much everything there was to say, thanks.
There's something else interesting about Opera: it has a couple of very useful "add-ons" which are like browser extensions. There's the email and IRC clients, the widgets, Opera Turbo and Dragonfly, but also two things that I think weren't talked about even though they are quite nice:
-Unite is some sort of sharing service. Basically, imagine a hosting service like MediaFire which would let you host and share files with friends, allow streaming of media files, and act in both ways (your friends can send you files to). Opera Unite does that, except the files are still "hosted" on your computer. You basically make anything you want completely accessible online, the only storage limit is your own HDD, and your files are perfectly safe at home.
-Opera Link is something I use a lot. As its name suggests, it links all your personal preferences (bookmarks, speed dial, notes, search engines...) to your Opera account and synchronises them with all your Opera browser. This is significant because Opera is a very big competitor, if not the biggest, in the mobile browser field. It means you have one "Opera identity" that you keep anywhere you go, you just have to log in to your Opera account.
These are the kind of features that will probably become available sometime on other browsers, but when they do, Opera users will already be playing with new toys.