Roughly Drafted on Exchange vs. OS X Server
Roughly Drafted does some great tech analysis.
You should read the whole article, especially if you're considering rolling out an Exchange server in an enterprise environment.
Users can easily fill up a 2 GB mailbox with saved email attachments on Exchange. The Standard Edition of Exchange only allows a 16 GB mail database, which can easily be filled to capacity by a little as 8 users.
Exchange can become a catastrophic nightmare to support once users start thinking of their Exchange mailbox as a file share alternative which handily sorts their huge attached files by date and message thread.