Hi bro,
While each M9 owner may have their own story as to why they decided own selling off their M9, the reason I sold mine was that I could not stand the dust/spots on the sensor. Having owned a sealed system with the Ricoh GXR, I didn't realize this was going to be much of an issue with removable lens systems. I had the M8 for a while before going to the M9 but strangely did not encounter any dust issues here.
With the M9 it happened about 6-7 months after owning the camera -- the spots were very obvious and since I didn't trust myself with sensor cleaning, I had to keep sending the camera back to Leica SG. Fortunately it was free since the camera was under warranty but the spots returned and I didn't like have to keep "healing" my pictures in Lightroom --after all, this wasn't something in my previous processing workflow with the GXR and the M8. So it was quite traumatic for me -- traumatic enough for me sell the M9 off. No fun lah, paying 10k for a camera and you have to hunt down and clean spots each time you import your images. I sold off the M9 and got a Hexar RF instead.
Now I shoot slides exclusively and send it to Lo Sheng for developing and Kingston for scanning. Slightly longer waiting time but I'm happy with the results.
FARI