"How good" is at least partly subjective, but a lot of people like them. (I'm assuming the question refers to the modern CV lenses in Leica mounts.) Some of their lenses have been put through objective tests for resolution and so on, and done well.

The superwides, of course, are pretty much unique---if you want a 12mm lens for a Leica, CV are effectively the only game in town.

-NT