Good and bad in no particular order....
Great for keeping feet dry while flying through filthy puddles, which happens most of the time, even when its not raining. (Think Three Rock)
Not great for heavy rain where the water will run down the legs into them anyway, even if using long tights/leg warmers over the sock (water resistant leggings or not). Top hem/elastic is very loose fit. Not breathable.
Are bulky, do have to leave an XC shoe a bit looser to allow for this.
Aren't the warmest without a liner, but even if they do get wet the water seems to warm up a bit like a wetsuit would, which is more pleasant then say, a wool sock that gets a fresh splash of cold water every time you hit the next waterlogged section.
Thickness is decent though if you whack your ankle off the crank or pedal now and then.
Even one puddle can leave normal socks/feet soaked for the whole spin, especially if wearing a mesh vented XC shoe, they are good for preventing this IMO.
Take ages to dry out.
Despite any adverse comments above, I'll be wearing mine all winter
Just don't expect them to be foot submarines!