One funny thing about Easy mode ... yes I'm on easy mode, you basically need to do it first and then playthrough on normal or hard, to play it "properly" - more on that in the inevitable review - you get a 0.8x points penalty, so its very easy to end up behind the curve in terms of aircraft.  You need to replay missions to "keep up" in terms of score for unlocking things, or be playing multiplayer.  That might be okay for hardcore fans playing missions a ton of times several times over and stuff, but it makes just trying to do the campaign start to finish as I'm trying to, very frustrating at points.