I've been involved with it from the time that IBM still published their internal table formats (../38 days) .. there were some major B/38 users who did all their system programming in it (e.g. Apple).. then with the AS400 software releases IBM went to the PI route to give a system programmer a 'consistant' interface and lots of MI code ceased to function .. For small things it has great utility and like all assemble level code can be tailored to precisely to the required task. Down side is like all assemble level code it can be obscure and difficult if the task is better suited to a higher level' language .. don't believe I have ever seen anyone try a 5250 screen with subfile in MI. The PI which assembles the code has few features of a 'good' assembler (like IBM 360 Macro Assembler) and the error messages generated are sketchy at best .. more often misleading.
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