Exchange High Availability

We've run across two large customers. One runs many Exchange servers with a limit of 800 users per instance and the other runs large HA clusters. The latter has many outside domains it must maintain. I'm wondering about the differences in the two approaches. It's possible the non-HA customer implemented before HA was stable with Exchange. I'd like to compare and contrast the two techniques and get an idea of where each one fits in large IT organizations