Amazingly good.
We bought this for a holiday trip from CO to TX and passed through a serious winter storm in the panhandle.  Everything arrived clean and dry.  I almost rated this four stars, my only complaint is the zipper can be a bit of a problem, especially getting around the corners.  That's more an issue in the extreme cold though.  At normal temps the material is more pliable and the zipper is easier to close.  But honestly I don't see any better solution and it does seem like it's breaking in.  It definitely beats having another hardbox that has to be stored for the majority of the year.
Rating: 5 stars