I use outlook simply because it got installed when I put Office 2000 on the machine, and I can't be RRRRd to learn how to use the Netscape mail program.
Outlook Express is fine but I like the functionality of Outlook- it's got all sorts of contacts stuff, a journal, alarm clock and when I can be bothered to sort it out, a fax facility (I think). The only thing that annoys me about it is the Newsreader, which is basically Outlook Express running without 'Express' in the title bar.