Not bad, but takes some work to get set.
First, this is NOT Gore-Tex, but a different Gore technology called Wind Stopper. The engineering is similar, but Gore-Tex uses 9x more venting holes that are much much smaller (too small to pass liquid water). Windstopper uses larger holes to better permit breathing and moisture to pass more easily, but small enough to act as an effective wind block. However, under pressure (i.e. wind) the fabric will allow water to pass slowly. This may or may not be a problem, but be aware that it isn't 100% guaranteed to be waterproof, only very water resistant.
That said, this product does the job. It will keep the wind off your neck and the skirt fits well into the jacket to seal regardless of the neck closure you have. Best of all, it does this in a very thin and comfortable material that you would never think had the ability to insulate.
My biggest complaint is more how this works with full face helmets. When you have the product up around the lower part of your face and put on the helmet, invariably the neck roll of the helmet pushes it down. I need to spend time tucking the fabric back up and then reaching in the eye port to pull it back over my chin. Not the easiest maneuver to complete. If all you are doing is running a few miles to shop, this will annoy you. However, for a longer ride, the comfort this product provides makes it well worth the effort.
When gearing up, take a couple extra seconds to be sure the tension is even from one side of your helmet to the other. I found on longer rides when I didn't get one side tucked in quite as much as the other that I felt I was holding my head at an angle. After time, this started to make my neck sore. A quick stop, readjust of the material, and all was well. However, that sore neck feeling stayed with me for a while. Now, I just make sure to rock my head from side to side to be sure there is even resistance and I've never had the issue return.
This product solves two of my most pressing issues:
First, it completely eliminates wind from getting into the jacket from the neck or helmet from the bottom. (assuming you have a well sealing full face helmet design) This made a huge difference in comfort on those really cold days. In fact, my entire head is warmer as the elimination of the drafts from below allows warmth to remain in the top of the helmet. Even with permanent rear vents, I'm plenty warm as you can't vent out air if there isn't any way for fresh air to replace it. This seal also seems to quiet down my helmet a little. I wear ear plugs, but every little bit helps.
Second, it keeps zippers, straps, etc off your neck, but does not add significant bulk. I never realized just how much friction I was getting until it is all gone. Getting my jacket zipper up tight was easier and I experienced less issues keeping the material under the jacket to allow the zipper to pass without snagging. Likewise, fastening my helmet was much easier than any other neck gaiter I've tried due to the thinner material. I do have to be sure to pull extra material up in the front so that as I cinch down the helmet strap, it can pull a little back. Not a big deal and you'll see what I mean as you use it. The helmet strap also helps to keep the product in place when you turn your head.
Speaking of turning my head, I don't feel I have any restrictions with this in place. I can't say the same about any other product I've tried.
This is a bit more expensive than similar products, but I've tried a few of the other designs. I disliked cycling balaclava products as the added bulk on my head tended to make my helmet more snug and never went on smoothly getting tugged and stretched as I put my helmet over it. Thicker material like fleece got in the way a lot and made it hard to zip up the jacket or fasten the helmet. Got quite frustrated with several such products. Worse, most fleece won't stop the wind entirely. You still get cold and you feel like you have a tire wrapped around your neck.
Hold your nose, pay the price, and revel in the comfort that this product will provide once you have it set and adjusted properly.
As for sizing, I typically wear a Large helmet (roughly 60cm around) and have a 17 3/4" neck. The recommended medium is tight going on, but once in place fits well. You could probably be a bit larger and still be comfortable in the medium.
While this product does require some extra time and effort to get in place properly, it does work very well, is comfortable, and does not impair your ability to turn and look.