I own both the 20 and 40 for my Canon 5D2. I posted something on this a few weeks ago when I purchased the lenses from a Montreal dealer. My only other lens is the 24-105.
The 24-105 is a great lens, etc. etc. but before moving to photographing digitally I mainly used a 35 mm Summicron and a 50 Summicron. The package difference is rather substantial and obvious. The 40 Voigtlander splits the difference in focal length and makes for a much less obtrusive piece of equipment. No real issues with focussing, and the dof scale is fine as long as you compress it by a stop or so. I actually went and made comparison images with the 24-105 at 40 mm. The VC was actually better, mind you, that was on a tripod and using LiveView for focussing the VC. There is much to be said for autofocus and IS.
The 20 is a neat lens. Yes it has the moustache distortion, but so does the 24-105 at 24 mm. Focusing is definitely not an issue, at any f/stop. The shade is a bit of a pricey metal ring but I bought it anyway. According to a supplier in France the Heliopan makes a 72 mm cap that fits over the hood. I lucked in and found a shallow round metal box that held some fridge magnets, once lined with a bit of velvet it is a perfect fit.
I am impressed with the solid quality of both lenses, as I am of a 21 mm that I purchased a few years ago to use on my Leica's. And, since everything else but the final print is rather moot, I very much like the results these lenses (and the 24-105) allow me to produce.
The Montreal (Canada) store's website is: http://www.leicaboutique.com/
I also visited a French website that tested both lenses: http://www.questionsphoto.com/article/378-...ngle-pour-canon