The (lack of) fan noise is a big plus.
Power consumption must be way down on the APX555 if they can get away with one fan, and that one is a thermo unit mostly off, or running slow. I wonder if AP could/would make a 'silent' fan kit add-on for the 555, with a few temp sensors, silent fluid bearing fans etc.
Fan noise on test gear is something I hate, and yes, I know they need to have the internals thermally stable, but some gear (DSOs) have such noisy fans, I end up only turning them on if I really need the digital storage aspect- I can leave analogue scopes on all day and they are silent, as are several other pieces of test gear.