/\ I love mainframe environments and certain middle tiers that run on them; in a sense that is real computing - no GUI, 1001 operations and keyboard commands that you have to learn and master in order to do anything productive, or even do anything at all.
Practically bulletproof compared to a win OS in terms of crashing or freezing - which is not to say a mainframe doesn't crash, it can in its parlance experience an ABEND (literally an "ABnormal END"); and less susceptible to viruses than Windows, although again not impervious to them - because of the command line interface I find CICS / mainframe computing interesting and challenging on a personal level because it really taxes the grey matter. CICS (acronym for Customer Information Control System) is actually middleware that sits between the z/OS IBM mainframe and your business applications such as financial and other operational business modules..
For the HIBC contract we have access to the BCG TSO/CICS environment in order to work certain customer financial adjustment requests, usually from the ministry. Requires us to go back and forth between CICS and the SAP financial module, which if nothing else mandates that you be sufficiently amped up on several cups of coffee in advance. Merely creating and staging such an adjustment file in CICS takes at a minimum an hour, sometimes two depending on the complexity of the request and how much information we need to physically gather and stage in the file.