I used an M Monochrome over a three day period when it was first released. My impression was very favorable in general. Personally, I found the images a bit too sharp and clinical. I think, like all digital files, your success depends largely on your post processing skills. My approach is to use a sync'd color management system and a standard workflow on color images that are converted to b&w.
Best regards,
Bob
CEO CFO EIEIO, Ret.