1. One Port exposed: 443. Most security administrators can handle this.
2. No client side software required. Okay a browser...
3. You can access it from anywhere, even at your parents house who might still be on dial-up!  
4. An almost identical feature-set to the full blown windows client.
5. Themes!!!

If you don't believe me try your current e-mail application then go here and sign-up for a free Exchange 2003 trial account and then see what you think.