I'll say more about it after I finish the holiday update to the Buying Guide, but what I will say about it now is that, of the current crop of new closed, around-the-ear portables, the MDR-1R is the one I'd pick for more critical listening (versus the M-100, M4U 1, MOMENTUM, and UE6000). I find the MDR-1R's sound very well balanced; and though its not neutral, per se, it is among the most even-handed I've heard in the sub-$500 closed, portable over-ear category.
Interesting comments. I'm listening to the M4U 1 now (through the latest Audirvana Plus w/Direct -> AQ D-Fly) and consider it to be a very "get out of the way of the music" type of 'phone. I'd wager that a large group of audiophiles would dislike it as it doesn't romanticise content -- recording quality is conveyed rather than nullified -- but it's for this very pure, unfettered presentation coupled with an un-hyped frequency response that I would lump it into the neutral/reference category and an ideal product for critical listening. Of course, the quoted statement could have been made solely within the context of portability and, in that, I would agree that the PSBs are better left at home.
Relavent: I have not heard the MDR-1R and, based on the comments made here, I am not actively seeking an audition per my own preferences.