I have the CV 15 and 21mm.

In both cases, build quality is pretty good. Both vignette pretty heavily.

I find the 21mm to be good, but not stellar (mainly due to some softness in the corners), though to do better, you probably have to shell out for Leica/Zeiss.
(I'm comparing the 21mm to the likes of a Zeiss SLR 18mm and a Leica R 19mm II, so have pretty high expectations).

I find the 15mm to be even better than the 21, though the previously mentioned vignetting is very visible.