If you are accessing and Exchange server, I'd say use Outlook 2001. Yes, it's in beta, but if you're accessing an Exchange server, you're probably currently using Outlook 8.2 (or 8.2.2, depending on how current you are), which is also known as one of the worst programs on the planet. Outlook:2001 (beta) works beautifully. The download is about 1/3 the size of the 8.2.2 download, installs quicker and better, and doesn't crash. I've been running 8.2.2 on one machine and Outlook:2001 on another for the past few days, and 2001 is far superior. All the great interface and appearance of MacOS with all the functionality we've been missing (read: Windows version).
But, to get back on track, if you have to ask, then you're probably not using an Exchange server. For the beta, at least, Outlook:2001 won't run on your Mac unless you are using an Exchange server. (Incidentally, Outlook 2001 [Windows] can be used with any type of mail server -- Exchange, POP3, IMAP. Whether the Mac version will eventually do this, I don't know, but if it did it would seem that it would encroach on the target market and functionality of Entourage.) Use Entourage. It's basically like Outlook Express with an added calendar, task manager, etc. I haven't used Entourage but the email client is basically Outlook Express 5, which is the best email program I've ever used.
With IE5, Outlook Express 5, Office:2001, it seems as though the Mac teams at Microsoft are not only finally getting things right, but they're doing it better than their Win9x/Me/NT/2G counterparts.