Yeah AC3 is pretty sweet. The "intro" took me three hours to complete and that was with some exploring and wandering (because who wouldn't want to spend as much time as possible as Haytham, the guy is a total badass). And once they gave me full control of Connor I immediately headed to the frontier to started wandering around and killing bears for like four hours.
As for the naval battles... fuck! It clearly isn't supposed to be a sim or anything, but god damn that shit is INTENSE.