Although I don't think success is a measure of quality by any means, I think there are other ways to attempt to measure how "good" they are. To a musician, having a serious conversation about it, this should be a no brainer. They are top-shelf musicians, talented to the max, whether you like what they do or not. BTW, I also love MT, and the one album I have really contrasts with my music collection centered around symphonic and prog rock and metal.
Edit: Oh, oh, oh...look what I just found. 1982 Playboy Jazz Festival at the Hollywood Bowl, California. Weather Report AND Manhattan Transfer playing Birdland...together.