Hi,
I to have this lens, as well as the 20 mm. I use both on a Canon 5d2. The main reason for the purchases was the size of the lenses -- quite a difference in the size of a 5d2 and the 40 VC compared to the 5d2 and the 24-105 zoom! I am used to focus with rangefinder cameras (film Leica's) so focusing either of the 40 or 20 with the regular ground glass would be iffy if not for the most useful centre focus confirmation point. I almost purchased the Canon 50 f/1.4 but the VC 40 is closer to the 35 focal length that I use most often. I tend to use my 24-105 at 24 to 35 and 105 with fewer images made at other focal lengths.
The image quality of the 40 compares favourably to images made at 35-50 with the 24-105
The image quality of the 20 is also rather good, even wide open given the kind of images that would require a wide open aperture; for really good quality the lens has to be stopped down to f/8 or f/11
If you read French, there is well thought out review at:http://www.questionsphoto.com/tag/227-voigtlander
Jean-Michel