i think it really depends on the cabinet and how many controls you are trying to fit in there. i find that the 24" panel is a little too tight for two adults. on my 2 player cabinet, i made my panel 30" wide and it was pretty comfortable for two adults. (2 sticks, 6 buttons each and a trackball in the middle). i have since redone that cabinet and set it up for classics with only a 4way, trackball and 3buttons, but i kept the panel at 30" so i didn't have to build a new box.
so, i say, 24" is plenty wide enough for a single player at a time, however it is too tight for two adults (unless you are both really skinny...which is not the case for me)
my 4 player panel is 45" wide, which is kinda pushing it for comfort.