I purchased these to use for whitewater canoeing and kayaking in the early spring when the water is still near freezing.  Overall they work fairly well.  I can step in shallow water with these and they keep my feet from getting saturated.  They also keep my feet warm, which is very important.  I can't speak to how 100% waterproof they are.  After wearing them my feet are always a bit 'moist' but they're certainly not soaked, so it could very well be sweat that is affecting the inside.  It's always warm on the inside though, and in the end that's what matters most.

If you get water over the top of the stock (say you step in too deep) these act as a bucket.  Your feet will be sitting in a pool of water until you take these off and dump them out.  On the upside, though, that water will warm up eventually.

The biggest downside to these socks is that they take forever to dry.  Literally days of hanging up.  If you plan on using them often, buy multiple pairs and rotate them.

Overall I recommend these for cold-weather kayaking.  You might also consider them for hiking but the seams look like they would be a little uncomfortable over hours of walking, so I'm not 100% sure on that.  They could certainly act as a water-barrier if warn over a more comfortable sock though.