This just about sums it up: Indiana Jones in a plane.
First off gameplay. It is just plane fun. Pun very much intended. While four player multiplayer makes it hard to see what exactly you are doing, in single player you the controls are simple and intuitive, and you can get into the game. Often times you will think you are in an open air cockpit, not sitting on your couch in the basement. There is plenty of variety in the planes, and the upgrades are a nice bonus. You will probably want to play through twice to upgrade all of your planes.
Second off, characters. You've gotta love them. They are funny, and likable, as well as being plausible. While not your everyday guy on the street, you can imagine someone in Nathan's profession acting the way he does. They also have feelings and stories. They don't always do things because they are smart, or analyze all the options like a monday morning quarterback, but instead act on impulse and based on their emotions or instincts like people would. I mean you can smack your head a few times when you think "Why did he do that?" until you realize that he is a sky pirate, not a detached nerd.
Third: the setting. I fell in love with the setting. An alternate 1930's setting where the U.S. has splintered, and whoever rules the skies rules all. You have giant zepplins with guns and aircraft hangers on board. Sign me up. It is not very practical, and never really explains why things ended up the way they did, but who cares, the setting rocks. Simple as that.
Finally, the games one fault. And I use fault loosely because it is not a fault per se, but the story doesn't quite hold up to the same level of excellence that the rest of the game does. It is good, and you will find it enjoyable, but it feels like something that was sort of slapped on as an afterthought compared to the rest of the game. Just a mediocre area.
Overall this is a great game and a must have for every x-box owner who enjoys a flying game, and even some who don't.