Wow. I am spoiled because in our firm the attorneys demand BB's and are willing to pay whatever it takes. Having to "sell" the idea of BB's to people is just.... well I feel for you.
Maybe if you are in a business where the difference between a 20 second push to device from receipt of an email on the server to a 15 minute "poll" via IMAP doesn't matter, but we certainly aren't!
BESX 4.1.7 on Exchange 2003: 65 Devices
BESX 5.0.3 on Exchange 2003: 2007 Devices