Julia White (Senior Director of Exchange Product Management) posted a nice post about email availability.
Keeping your email up and running is mission critical. While other business applications can go down without much notice, when email is down for even a minute everyone in the organization notices and wants answers. As such, Exchange admins have long had approaches for ensuring email availability through various approaches. This is also why Exchange Server 2010 and Exchange Online provide built-in high availability - nothing more is required to ensure email high availability.
Customers that have upgraded to Exchange 2010 have already seen the benefits of high availability tools that we have built right into the product.
"With Exchange Server 2010, we've been able to eliminate the third party hosted disaster recovery solution, which saves about £12,000 (U.S.$19,560) per year." - Steve Derbyshire, IT Manager, NEC Philips Unified Solutions
"With our new solution built on Exchange Server 2010, we realize consistently high availability and the site resiliency with very little maintenance and administration. We also expect to reduce our RTO by 20 percent." - Dan Evans, Manager of Messaging and Collaboration, Morgan Keegan & Company