I have been looking for a solid e-mail client for quite some time. I seem to have found one that I really like, MS Entouragge for Mac OS X. Entourage is much farther ahead of Outlook in that you can not only flag an e-mail, but set a reminder in your calendar to come back to that e-mail at a later time. The ability to set categories in addition to rules and other elements is a great help. I find it easy to use and work with.
The competition has left me cold to some degree. Netscape 6(plus) is light years ahead of version 4 (did not allow pulling from multiple e-mail sources), begins to really choke with a couple thousand e-mails in a folder (the downside of being on some wonderful listserves that are full of great information). Outlook Express does not permit archiving. Outlook on Windows has a muddled interface that works well with Exchange and that is not completely a plus.
In all I am very impressed with MS Office on Mac OS X. That is my "productivity tool" of choice. It loads faster and the application does not get in the way of doing what you want it to do, although "Clippy's" cousin is alive and still annoying.