Add insurance and airline industries as big users. Mainframes are not going away. There is a general shortage of qualified folks to support and program them, and many of the folks who are qualified are aging. So yes, by definition having mainframe skills is marketable and salaries are attractive. On the (potential) down side, mainframe jobs exist almost exclusively in large, staid organizations, and mainframes are by and large doing the most pedestrian of data processing tasks. While there are exceptions, going mainframe means being comfortable in large, technologically conservative, bureaucratic organizations working on by and large mundane problems. You don't get a lot of time working on cutting edge technology (although some folks are doing interesting things with Linux as a z/OS VM). In exchange, you'll be well paid and work in very mature and experience operational environments.
