You can fly it around the map and you can land on most (but not all) roads. The only real actual functionality it has, however, is that there's a few (very small) areas of the map that can't be accessed any other way than by flying to them, and there you'll find some hard post-game fights, basically. Because it comes not just after the story but after you've completed most of the side quests (as you have to do many of them to unlock it) it doesn't really have that much functionality, which is a little bit of a shame. It looks cool as fuck though.