10 - Simply because there's no combat flight simulator that's better.
There's no feeling better than getting behind the stick and blasting an opponent who is on your friend's tail. Seeing his fuel tank catch on fire and then explode. Or diving on an opponent and sawing off his wing before he even knows you're there. There's an insane amount of planes, vehicles, ships, maps, etc.
The only problem with this game is online, which requires a third party program to do. Networking is great otherwise. Takes a few seconds to download a mission, much better than games nowadays.
Singleplayer is just as fun. There's about 40+ campaigns. Each is completely randomized every time. How well your nation does in the war is dependent on how well you fly. If you shoot up a train then the enemy's forces fail to arrive to defend the front lines. You'll get awards, promotions, better fighters.
I challenge you to show me a better combat flight simulator. Otherwise, this game deserves a full-out 10.