I’ve been using this for years now, since version 2.x. It is available both “free” as adware, and in an uncluttered version for around $40.
The real advantages over IE are in the user interface, usually the thing that Microsoft gets right. But opera has a tabbed MDI dispay, which makes it much easier to keep track of multiple windows; it has a great many really useful keyboard shortcuts (keyboard plus and minus to zoom; ctrl-backspace to jump up a level in an URL; much more). there is an excellent teeny search bar in the corner of every window. It is possible with one keystroke to switch stylesheets and use one you have set up for your own legibility (that’s how I can read what I’m writing now).
There is a primitive IM client that lets you send and receive ICQ messages (not chat) that I find quite useful, and a mail client I have never used and can’t talk about.
Fundamentally, though, Opera is nothing but a browser, and a damn good one. The only thing that I can think of which IE does better is the display of history folders. The only persistently irritating bug is a reluctance to reload linked stylesheets when they change, which makes it a pain to test web design changes with. But otherwise I use it because it is clearly superior to every version of IE up to 5.5 (not tried 6); and, it goes without saying, Netscape 4.x.
Anyone interested in alternatives to Microsoft should certainly check it out. Runs on Windows, Linux, Psion/Epoc and maybe other things I don’t know about.