Allied force is one of the best games ever made for a PC.
However, its not for everyone.
Good:
1) Good support from the developers.
2) Dynamic Campaign.
3) Realistic.
Bad:
1) DirectX7 based graphics engine (shaders? None found here, its GeForce2 compatible!)
2) Inconsistant quality, when things work good, they are outstanding, other things barely functional.
3) Campaigns can seem a bit whacky sometimes.
The reason why I say its one of the best games ever made: They built a realistic simulator ontop of a complex strategic war game. Think of a real time strategy game where you have land, assets, production, time to repair, time to resupply, radars, defensive units, offense units, etc. The dynamic campaign in Falcon4:AF is like a realtime strategy game.
The game is going to set goals for your campaign "Capture North Korea." In order to do that you need to capture the 2 largest cities, Wonson, and Pyongyang. Now, as F-16's you are based in South Korea. So the game starts calculating how to achieve the goals.
First things first, we need recon, and see where enemy troops are at. So launch some recon helicopters. We need refueling planes in the air for our aircraft. We also need AWACS aircraf to spot enemy aircraft. We also need fighters patrolling these assets so they don't get shot down. We also need to start scheduling transport missions so fuel and supplies don't runout at certain bases.
Now that we have the recon, schedule some attacks, enemy tanks keep coming over this bridge, so lets schedule a strike mission to take it out, now they have to find other roads to travel to get across the border. Enemy fighters keep coming in, so we need to take out their airbase. In order to do that, we need to schedule SAM strike missions to take out anti-air missiles on their side. Then we need to send in escort to take out enemy aircraft. Now we've punched a hole through their defenses. Now we need strike missions on the runway. It's all coordinated, and planes converge on a target at the right time. Once you strike the airbase, the enemy has repair units to fix the runways, now the game has to schedule attack missions on that repair unit to prevent runway from being fixed. In order to prevent repair units, you have to bomb their factories, etc etc etc.
Entire war is being run in realtime, even if you are not even flying a mission. You should literally play the entire game without even flying a mission, just watching how the war unfolds on the 2d map. You also have the ability to command tanks, and schedule flights, etc.
Thats the cool part. The fun part is jumping in and doing the fighting in game flying your F-16... As you are flying your mission, all the other missions and the war is still happening around you. You can see tanks fire shells at each other, SAM missiles shooting down other aircraft doing their respective missions, etc.
This really brings out the immersion, you feel like you are a single peon doing your part in an entire war. If you get lucky and shoot down 6 enemy aircraft on your mission, you will notice it on the later missions, as their air patrols won't be as thick. You can obliterate their aircraft through attrition or bombing all their airfields, and you will have air superiority and you won't have any enemy fighters against you in future missions.
If you are really interested in flight sim that tries to replicate realistically being part of a war, this is the game to get. But be warned, the graphics are basically year 2000 quality. You will probably want to puke at the quality of th graphics. However, if you can get over it, and try to learn the game, you will have a ton of fun.
This video should give you a feeling for it:
http://www.youtube.com/watch?v=eF1RJwg902A