I own four units myself, a Sony MZ-R55, Sharp MD-MT190( my "in the field" unit), Sony MDS-JB940, and the Cadillac of the portables, the MZ-RH1. They have so many advantages over the MP3 players, the main one being that they were pretty much designed to record live as evidenced by the fact they all have mic inputs. Except for the big home deck that is. Back when MD was still a viable medium you could get great deals on media and its re-recordability factor was a real big plus. I'll never run ouit!
I agree that in listening tests it beats out MP3 too. I think Sony did a big booboo with its insistance on the SCMS system. I mean you couldn't even copy the recordings YOU made.