CR, certainly many of us here would agree with your assessment of the 500/590 variants. Beyond reliability two features set it apart from any other shotgun for me -- the natural ergonomics of the controls and the ability to combat load it on the move.
CR, I'm an old school guy and career military special ops vet. The simplicity of the system is it's beauty. The only "real" modification to my Mossbergs has been the opening up and smoothing out the loading port opening with a file. By knocking off the sharp edges and extending the ramp on the front of the port you can thumb in shells with ease and with speed. I typically combat load two shells at a time.
This port modification coupled with receiver mounted velcro shot cards also allow immediate changes in ammo types as the situation dictates. Typically carry several cards with a mix of ammo types on each based on the anticipated situation.