I used to have a CV 25/4 in M mount (RF coupled). Very good lens: sharp, contrasty, rich color rendering, compact, easy to use.
But I found myself using the Canon 25/3.5 more, as I just like the vintage rendering better, especially in b&w -- so I sold the CV.
Notes on the Canon:
1. It's built like a little tank. I bought mine on eBay, and it arrived thrown naked (no caps, case, nada) into an unpadded manila envelope -- no damage.
2. The aperture ring rotates with the focusing barrel, so if you tend to constantly adjust aperture while shooting, it can be a bit of a pain.
3. The optical formula is a Biogon derivative, and it protrudes deeply into the camera body -- so there will be likely be light fall-off and other weaknesses in the corners on digital M cameras.
4. Front lens element is deeply recessed in the lens body, so you don't need a hood, unless you stick a filter on the front.
5. Filter size is 40mm, which isn't so easy to come by, but a 40mm-40.5mm adapter ring is easy to come by, if need be.
6. The glass elements are tiny, so take a close look with a bright light, or you won't see scratches or haze with the naked eye in normal lighting.
Cheers,
Ari
EDIT: As noted below, the Canon is a Topogon, not a Biogon, formula.