I bought the 40mm pancake lens with my 6D, and I like it a lot. Sharp to the corners wide open, nice color and contrast...not quite "L" quality, but the sharpness pretty much is. Distortion also seems low. Like others who have reviewed the lens at the various retail websites...I do find it to be a very tight fit when mounting. As it begins to feel tight, I slowly unscrew a bit, then twist tight again...and it feels like it's not wearing the mount out too much. I've mostly used other lenses with the 6D, but plan to use the little 40mm quite a bit more.
I also agree with the digital picture website's review, where they compare the 40mm pancake directly to the 50 f/1.8 ii. The 40 absolutely kills it in most every aspect other than f/stop. I'm glad I sold my "nifty fifty" over a year ago. I don't miss it. My Voigtlander 58mm f/1.4 replaced it. It is almost beyond "L" quality...but it is a manual lens.