I have the closely related HP 54112D scope. Although it works well (after I repaired it), these units are representative of HP's first plunge into digital scopes. The user interface, in particular, can be quite annoying. Rather than having dedicated knobs for often used functions like vertical V/div and horizontal time/div, everything is push a button, spin the dial. These units are big, heavy, somewhat limited by their equivalent time sampling, very low update rates, 7-bit digitizer, small memory, etc.
I view using these scopes as a rite of passage. Sort of like in a calculus class, they make you find the derivative of y = x^n the long way using just the definition of derivative, before teaching you to appreciate the simple formula: dy/dx = n x^(n-1). Everybody has to do it once and everybody should use a scope like the 541xx series once so that you can appreciate how much better a modern digital scope can be. If I recall, the specifications are nothing special (for example, 2-3% vertical accuracy) and aren't going to beat a modern scope in my opinion.
But since you got the 54100A at a price that's hard to beat, have some fun with it. It is not an easy piece to work on, so hopefully it will stay working.