Sony did bring out commercial albums on MD, but they were about the same price as CDs and the range was a little limited especially at first (artists on Sony's record labels) but it did expand later.
The original MD format is better sound quality than the Long Play MD, but I've not heard the new HDMD (something like that anyway).
MD's are still in use in broadcasting, both for portable recording and in the studio.
One main advantage of Minidisk is that all players have a 15-30second memory so if you hit a bump in the road, you *shouldn't* notice the music skip.
I used to use my portable MD in the car with an adaptor, but these days I have a Sony MP3/CD unit which is great, 10 albums on one disc. But if you hit a bump really hard, you can get it to skip.