I agree with Lee and it all depends on how you use it. If you have 100 individuals access Exchange from multiple devices then go with user CAL. If you are a manufacturing environment running 3 shifts where 300 individuals over a 24 hour period access 100 mailboxes from 40 computers then in this case it would be advantageous to buy 40 Windows Server 2008 device cals and ditto for Exchange.
kola12
It depends on how users will access their mailbox. If your users have access to their mailbox with more than one device (pc, mobilephone, laptop) then cheaper are buy 100 users exchange CALs. When users have access to their mailbox with only one device then buy 100 device exchange CALs. With Windows CALs are the same history.