Strengths: These do a great job of keeping you warm in dry sub freezing and might keep you warm in wet under 45 F conditions if you don't get a soaking we chamois (see weaknesses). Only product I could find at the time that offered this level of warmth and water resistance in a single-layer tight.
Weaknesses: WET: I can't say that they'll keep you dry though, there is a 4-panel seam forming a + right in the center of the chamois, right where the fabric is most stretched if you use drop bars and right where rear-wheel spray hits hardest and/or drips down. Stretched seams are not waterproof and with the absorbent chamois underneath, your keester is soaked and squishy in just a mile of moderate rainfaill.
FIT: I'm 6', 170lbs, normally built cyclist. I typically wear Med and ocassionally Large if length will be an issue. Based on their fit-guide, Med would be too short (recommended for riders up to 5'9") so I went large. Basically, they are made for someone with a much larger butt than mine - I could have ridden with an extra helmet tucked into the back of my pants. Additionally the knee was positioned too high (kind of strange-sounding, I know) so that the lower leg was a little loose and the upper wanted to pull down as I rode. This makes the chamois pull away from the skin and makes riding quite uncomfortable. So if you have a shorter thigh, longer shin and bigger butt than I do - they'll fit perfectly.
Admittedly, a perfect fit might resolve the wet chamois problem too, but I can't confirm this.