We used Heartbeat as a high availability solution for one of our customer at web, database and file system layers. The web and database cluster uses heartbeat to check the availability of the primary node, and to handle IP fail over to a peer node in case of a malfunction. The shared file system is configured with NFS, which is stored on a DRBD partition. Heartbeat manages the mounting of the DRBD partition and the start up of NFS. In case of failure, the services are moved to a peer node. The system was design for high availability at each level and is expected to offer a reliable service to over 1500 users.
In another scenario, we use Heartbeat in combination with DRBD for Mysql highly availability. This setup has been live and working for a long time and Heartbeat is managing the fail over in a professional manner.