Uh - oh! Somebody roused the BEAR! RUN!!!!
Nothing rouses the Bear more than the quality claims of minidisc users! I have the original 4-track, the MD4 (with 1st Gen. ATRAC), and yes, it is better than cassette. But anyone who can't hear the difference between MD and CD or 2" tape needs an appointment for a hearing test. When I bought the MD4, it was the only choice available for a digital 4-track. The next step up was an ADAT, but they were rather expensive back then and needed a mixer.
Now, with a plethora of 16 bit hard disk recorders full of effects and routing, there is absolutely no good reason to buy a MD Multitrack. The 2- channel versions will enjoy a niche (as mentioned - field recording, backing tracks, etc.); but my little MD4 has quickly gone the way of my old Tascam Portastudio Model 144!
Bob