I shoot, on average, 50% of my images with a Leica M2-S and Tri-X, so the M9 Monochrom seems interesting. But then when I looked at the samples on Leica's web page, the one issue that has always concerned me about a digital B&W sensor looks like it hasn't been solved.
I use color filters with my M2-S to get particular effects in a shot. I know how Tri-X responds to each of my filter colors. I've never been convinced that a digital sensor would respond the same way.
As an example, here is a shot from Leica's web page of the M9 Monochrom. Here they used an orange filter to add dramatic effect to the blue, cloud filled sky:
And here is an example from one of my Tri-X shots of a similar situation where I used a similar filter to get a dramatic sky effect:
While the orange filter in the M9 Monochrom shot darkened the blue sky, it also darkened and muddied up the white clouds. With Tri-X film, the filter darkened the blue sky but left the white clouds a puffy bright white.
Best,
-Tim