Microsoft has released an interesting whitepaper about large mailboxes on Exchange 2010.
Giving your users the ability to store more e-mail has many advantages. Large mailboxes keep e-mail on the Exchange Server instead of allowing it to be scattered in Outlook Data Files (.PST files). That helps reduce the risk of data loss, improve regulatory compliance, and increase productivity among both workers and IT staff. The main barrier to implementing large mailboxes is the perceived cost and complexity of storing large amounts of e-mail data. Microsoft® Exchange Server 2010 is specifically designed to overcome these barriers. This paper discusses how Exchange 2010 enables you to give users large mailboxes without breaking your budget.
Download and read the Whitepaper here.