I agree that the windshirt was one of the hardest things to get used to in the UL kit. I got a montane featherlight, which has been very nice so far. I find that it only sheds a very light mist, in which case I'd rather go with just my base layer.
The other day I wore the windshirt in steady rain, and it wet through almost immediately. However, it kept me warm enough that hiking in the rain was not uncomfortable, and when the rain stopped the windshirt dried off almost instantly (it holds almost no water at all when saturated). I was able to keep my rain jacket dry for camp, rather than sweating through it while hiking. So far, I think the windshirt is a good investment.