LTM lenses open up wonderful possibilities on the Leica Monochrom. I've got a first generation Leica M9M and regularly use Nikkor-SC 5cm f1.4, Nikkor-PC 8.5cm f2.0, Nikkor-QC 13.5cm f3.5 LTM lenses and Canon 28mm f2.8, 35mm f1.8 & 35mm f2.0 LTM lenses with it. I like the way they render with lower contrast. The Canon 28mm & 35mm f1.8 are a bit soft around the edges wide open, but even that can give a nice vintage feel to images.
I guess I've been lucky as my Nikkor-QC 13.5cm f3.5 lens focuses wonderfully with my Leica M9M:
Leica M9M w/Nikkor-QC 13.5cm f3.5 & Red #25 filter