MS Outlook is a virus-spreader waiting to happen. I don’t use it myself anymore, however there are many people who do, either because they don’t know any better or they need it for work or whatever. There are a lot of tools out there which only work for Outlook (Palm mail sync for one) which may lead you to decide to use it.
One of the biggest problems with Outlook is the extremely broken way that it formats replies. I came across a freeware program called which will automatically fix Outlook’s broken quoting.
(Personally I love Opera’s M2 mail program far too much to ever use anything else. M2 can import mail from Outlook Express… for Outlook you have to first import into something like Mozilla mail and then import it from Mozilla into M2… hopefully this will be fixed in a later release.)