I have a Gore-Tex Exchange bivibag, complete with two poles to keep it out of my face. The first night i slept in it, I had rain and yes, you feel the rain falling. Hence I thought it was leaking but it didn't and I was perfectly dry. I than slept sooooo comfortable, being warm and dry without the rain being able to do anything :D.
I almost never take it with me though. Firstly the 2 lbs 11 oz wieght is just too much and secondly it doesn't work when you have to get in or out in the rain. That goes something like this: first build up bivibag; than get my downbag out of my pack and its waterproof bag; by the time i have put sleepingbag in bivibag it'll be wet; than undress (now i'm wet as well) and get into both my bags, being wet. It just doesn't work.
Only time i do use it is on solo winter trips (don't worry, i go hiking very moderately, no avalanch risks or other possibly live threathening situations). It's perfect for those trips. It upgrades the temp. rating of my sleepingbag with atleast 10 degrees and there's no rain in winter so i don't have to worry about me or my sleepingbag getting wet.
Gore-Tex Exchange is a gas permeable fabric. While normal GTX only lets watervapour permeate, GTX Exchange also lets poisonous gasses like CO out so i don't suffocate in my bivi. I had one night where the outerfabric of my sleepingbag got quit wet, but i found out a week later that i should keep it completely zipped closed and than it does a good job in breathing, besides i also get condensation on my sleepingbag sleeping underneath a tarp in early spring when there's a big temperature difference between daytime highs and night time lows so you're never completly save from condensation.