I use entourage to connect to my work exchange e-mail. It works fine, but it is slow to update. An update of the mailbox takes 2 minutes (which is annoying if it happens every ten minutes,) and a full restore of my 2.98 GB database takes 6 hours! However, its the best way to access exhange e-mail from my mac.
I actually just started using mail.app for my personal e-mail because of what I consider to be an amazing addition in leopard: quicklook attachments. It is sooo much better than how entourage deals with attachments (not only does it open the associated program for each file, but each time you open an attachment, it saves it to a temp folder that continues to grow until you remember to clear it out.)