I have a pair of ancient Rigida Grizzley CSS rims that are still practically unworn, with flat braking surfaces. I have rebuilt one onto a new hub after the original hub expired!
To answer the question, braking in the wet was an issue in the first 10, 000 miles of use, but not since. They improve enormously when they have worn in. I used the recommended Swisstop blue pads at first but did not find them any better than cheap Clarkes pads for alloy rims. I have since been told that Koolstop salmon pads work well. Whichever pads you use, they will last much longer than on plain alloy rims.
My tourer now has these old rims and they look set to go on and on, and wet weather braking is fine. But to get over the initial phase safely you may be best advised to have one non-CSS rim, probably the front.