I stuck with Entourage for almost 10 months, but no longer. There are several reasons for this: the Mac apps are easier to use and simpler, I like the threading feature in Mac Mail which shows all of the emails grouped together that were replies to the original email or replies to replies, The Mac Calendar is soooo much more aesthetically appealing, I can sync the address book and the calendar to my ipod (I gave up on my Treo 600 several months ago for a smaller simpler Samsung and really started missing the calendar functionality)
... and the coolest thing (drum roll here), you can bounce spam email messages :)
"To discourage someone from sending you email, you can reject, or "bounce" messages from that person, as long as the return address is valid. When you bounce a message, it appears to the sender that your address is invalid. This is useful for rejecting mass mailings from companies whose mailing lists you've been added to after buying products from them."