These waterproof socks are amazing! I wear mine cycling during the autumn/fall, winter and spring - whenever getting cold feet would ruin a ride. They are no thicker than a chunky sock, and far warmer in damp conditions. I use these with normal cycling shoes, in conditions well below freezing, whilst my fellow riders switch to bulky winter boots, woollen socks and neoprene covers. They are also really useful if your shoes haven't dried out from a previous day's activity, or from the morning's commute in - putting warm dry feet into wet shoes is no longer a problem.
Yes - water does end up inside the sock, but this has come from above the cuff, either from stepping in water that's too deep, or from splashed water running down your leg. It's not a problem - the water soon warms up and I've never suffered from cold feet in this situation.
Downsides? Feet can get pretty sweaty in there, so airing them out after use, and washing them and drying them carefully is a must. I do machine wash and low tumble dry mine, but it's important to turn them inside out between the wash and dry; water can't escape from inside the sock during the spin cycle, and they stay soaking wet. If you leave them that way, you'll come back to stinky damp insides. Turn them inside out and all seems to work well.