Muck Boots are pretty much just a more comfortable version of the traditional high rubber boots. They will still give you the "full booter" if you go over the top (so far I haven't managed to sink that far). Good for cold water conditions or wet portages, if used in the summer I think you'd want to slip them off while paddling. Probably not very good for wading in rapids, "full booter" + strong current = bad combo.

On the plus side they are really comfortable, durable and easy to get in and out of. Oh yeah, they float too.