Hi,
I use MC primarily as a music player, although I do use it for video also (because it has the best sound!).
I have previously purchased 16, 17 and I now have 19. Each has offered me sufficient reason to upgrade because of enhancements to the audio side of things that have all made a real difference to the quality of my listening experience.
What prevented me upgrading to 20, and is now having the same effect with 21, is that nothing significant has been 'improved' audio-wise, since 19 - at least of which I am aware.
I understand that there is a limit to how much 'better' things can be made, especially with digital audio. I think this limit was more or less reached in MC19 - it has everything I need to make it the 'perfect' audio player, and I have been happily using it every day for the last three years, without a single problem, or any feeling that there are things I want that it cannot do.
On the other hand, I like it so much that I do feel inclined to support JRiver, so I always consider upgrading simply to help fund continued development and support.
So, the point is this. Is there anything sonically significant (I only ever play APE CD rips at 44.1khz) that 21 does that 19 didn't?