Here's their page on the service for the Blackberry. http://www.exchangemymail.com/blackberryfeatures.html
I'm sure Dave will chime in here and answer your question. I called their number and spoke to him live. He answered all my questions and concerns.
I've only been using the service for two days but already I see the following benefits:
1. The use of the MDS proxy.
2. Faster internet browsing.
3. The ability to use third party applications which rely on data.
4. Wireless synchroniztion of calendar, contacts, tasks and notes.
5. Outlook web access (access your Outlook from any computer anywhere.)
6. Ability to use folders within Outlook on your Blackberry.
7. Free upgrade to Outlook 2003.
8. Free e-mail account.
9. No need for desktop redirector
10. Triple DES encryption technology.
11. Ability to specify "from" e-mail address on outgoing messages.
12. Real live technical support only a phone call or e-mail away.
13. Immediate e-mail delivery.
I feel I was previously only using the Blackberry to 75% of it's potential. Like something was missing. Now with the service it's being used the way it was designed to be used.