Fair question by Pier. Reasonable reply (albeit harsh) by PG. Certainly a search will connect you with several threads related to your question, so indeed you should give it a whirl.
I don't think I've ever added my 2 cents in any of those past threads, so I'll chime in now.
I'll preface this by saying, I'm no expert. I'm only an enthusiast who is stumbling through, making tons of mistakes.
I like my MD8 over the cassette format because it is far cleaner a recording than tape. No hiss or muddiness. I guess there is some loss of fidelity due to 5:1 atrac compression, but I cannot tell and many others cannot tell either. If your happy with cassette, you'll be beyond your wildest dreams with MD.
The Yamaha has many nice features too. Phantom power for condensor mics, XLR inputs on channels 1 & 2, decent mic pre amp, built in mixer, etc. In addition, it operates almost as simply as your 424 MKII.
MD disks are kind of expensive--about $15 per give or take $3 depending on where you buy them. Now only Yamaha is making the multitrack MD recorder, so I guess there's the risk that this format may go away someday (soon?).
I'm sure I missed some important points, but in a nut shell, I think it's better than cassette and easier to use than other all-in-one recorders. If your bright enough to operate one of the other all-in-one hard disk records, the fidelity is probably much better.
Just some random thoughts. Good luck making your decision.
DHN