I must say that I think is is, or was, actually a quite good and robust system (although I prefer working on Unix).

We have a huge monolithic customer management, phone provisionning, rating and billing, and accounting application running a cell phone client base of 35 million customers on 7 VMS clusters.

However, HP has announced that it will discontinue suport within about 5 years, so we will probably port that application to Oracle and Linux. Migration from VMS will be a pretty large project.