I just got my M4 CLA'd by Youxin Ye from Yye Camera out of Canton, MA and I gotta say, the job he did was incredible. I was lucky enough to spend the time in his shop as he tore my camera down and adjusted it. It was a great experience, and I never expected him to walk me through the majority of the process. At $150, it wasn't particularly cheap, but it's much cheaper than most of the Leica CLAs you can get done, and the turn-around time was two and a half hours! Nothing like getting a brand new feeling Leica in a matter of hours!
Plus one for Youxin Ye..... hes the best value out there for Barnacks. I have never used him for an M but I hear he is great with them as well.