BlackBerry Enterprise Server also has high-availability features that Express lacks. For instance, you can configure primary and standby servers for automatic and manual failover -- which could keep downtime to a minimum when there's a hardware problem or during server upgrades. (There are no additional licensing fees for servers running in standby mode.)
Working in concert with failover, BES 5.0.1 adds system health checks. For example, you can create a certain performance threshold. If that measurement is exceeded, the failover to the backup server automatically occurs.
Both flavors of BlackBerry Enterprise Server do a very good job of providing BlackBerry users with secure, wireless access to email and documents behind the firewall, and the Web-based interface minimizes the workload of IT administrators. For personally liable BlackBerry devices that only require access to an Exchange server and where a basic set of security policies is adequate, Express will do the trick. But when your support staff has to manage thousands of devices or when email to mobile executives absolutely positively must never stop flowing, then BlackBerry Enterprise Server is the only choice.