The MZ-R50 is the "classic" in the Sony line. The Sharp "classics" are the 821/831/702/722 (all very similar except for the case).
We own an R50, 702, and 722. Overall, I would say that the R50 is one of the best MD portables ever. There are really only three complaints I have against the R50:
1) End Search. The stupidest "feature" ever put on an electronic device. People have been complaining to Sony about it for years, but the most they do is that the very latest models (R900, etc.) allow you to defeat it, but it's still there. But this is more a criticism of Sony models than the R50.
2) No on-the-fly record level. Again, a Sony problem, not an R50 problem.
3) It doesn't have a wonderfully big (and useable) jog dial like the Sharp 722 -- but neither does any portable but the 722
And, in fact, the R50 actually dues have a real thumb wheel that rotates 360 degrees, which is better than any other portable save the 722, so, again, not really much of a complaint.
It has a very good headphone jack, a true, dedicated, "I have my own circuitry" line-out jack, and is built like a tank. Battery life is actually very good, considering when it was released (it was one of the first devices to use a Lithium Ion battery. You get 7 hours of playback off the built-in battery, 12 hours off of two AAs, or 22 hours if you use both together. For recording, it's 4 for the LI, 5 for AAs, and 12 for both. Those numbers are still quite good, even today.