See: http://smg.photobucket.com/albums/v313/RiceHigh/5Ds/40mm_Pancake/EF40STM/
Do note that there is heavy asymmetric vignetting at the right side of my pictures which can be seen even when stopped down, although the effect is not as dominant.
A quick comparison to another previous brick wall test by another user of the same pancake lens found that the asymmetricity can be found on his samples, but surprising at the left side instead for his samples!
http://forums.dpreview.com/forums/read.asp?forum=1029&message=41817473
My that copy of the lens was replaced shortly afterwards anyway. The replacement seems to have better symmetricity on vignetting at all apertures and from f/4 and onwards the effect has been far less obvious.
After all, it seems that manufacturing a pancake lens is not as easy as imagined and even tighter QC may be required. I guess and hope that Canon has learned the lesson.
I bet Pentax had the same mistake when they first produced the DA40/2.8 pancake lens, as what I can see and compare the test results of the first sample of the lens against the latest, i.e., the DA40/2.8 XS. The XS lens looks far more symmetrical now. Nonetheless, this maybe due to sample variation anyway.
Btw, if the quality of the EF40/2.8 lens is not good enough, just buy a EF400/2.8L, which can be used as a big stone for throwing during a riot! ;-D Read this Reuters story:-
http://blogs.reuters.com/photographers-blog/2012/05/16/saving-the-canon-400mm-f2-8/