Hi Aboud,
Others with more long term experience (with this combination), can chime in here, but in my limited trial with both this lens and the M9 left me with mixed feelings. The lens on the M8 is excellent, excellent center sharpness (resolution), good corner sharpness and moderate light falloff in the corners which could easily be delt with in post processing. Since I had used more than 3 samples over the years with this lens (some original screw mount and one M mount version), there is a moderate amount of sample to sample variation in terms of performance. I won't comment on barrel distortion as it was not something with this particular I was concerned about.
Here though are the issues with using the lens on the M9..which also applies to some other wide angle lenses to varing degrees. The Red Edge problem and the VC 15mm suffers from this possible more than any other lens except possible one particular older Leica 21mm lens. This issue is there in abundence and so far even with the new M9 firmware, it hasn't been resolved. There are a myriad of ways to code it and using the code for the non asph 21mm f2.8 Leica seems to help. The only real workable solution has been to use "Corner-Fix" in a post processing workflow, which if apllied properly, can eliminate most of this isssue in a wide variety of circumstances. Of course shooting strickly in B&W is not an issue. Some have applied a gradient filter in C1 (instead of corner fix) with certain other lenses that show Red Edge...but not sue how well that approach works with this lens. Anyhow, I'm sure there is more that can be added by others both here and on the Leica users forum. Hope this info is of some help.
Dave (D&A)