I, too, have a 28S that rarely sees the light of day. Instead it is my older, and much less advanced, HP-10C that gets occasional exercise.

The 28 and 48 series sat in a kind of awkward spot between pocket calculators and "real" computers. The programming language is essentially Lisp turned backwards, but for any task where you actually need that capacity, it must have been easier, even at the time, to just use a PC (in the wider sense, not just IBM PC and clones). The 48 had better communications facilities, so I imagine it might have been useful where pocket carry was actually necessary (it seems to have been popular with surveyors, for example, and I have seen them hooked up with a cable to a total station).