Generally I prefer WWII themed-dogfights on PC or PS3 as compared to "set and forget" missile combat. Yet somehow ACI is quite addictive and challenging in its online mode. Online is co-op only with two teams competing against AI to get the most points and win the match. Players are rewarded with points to buy and/or upgrade planes and weapons and gain fuel rewards.
Fuel is the key to playing ACI. You gain fuel (slowly = one unit of fuel per 4 hours) over time or you can earn it by completing in mission challenges or you can buy them for real money (ie depending on how quickly you want to rank up and or unlock new planes the game is not as "free" as you might think). This fuel is essentially on a use it or lose it basis as it maxes out at 3 fuel until you start using again triggering the 4 hour countdown for the fuel to be replaced. "Fuel stocks" which are pemament and build up (until used) are earned by in-game rewards for completing sortie missions or challenges (ie targets detroyed, points earned or specific missions undertaken in specific aircraft, etc).
All is explained here: http://acecombat.wikia.com/wiki/Sortie_Fuel Alas without fuel you cannot undertake sorties (ie missions) and each online (~5 minutes each) or single player campaign mission consumes one unit of fuel. So far and going forward I will keep on relying on free (or earned) fuel as ACI "lives" on your PS3 you can always drop in for a few matches between other games.
I usually play two sessions (each using 3 fuel each time) per day during the week. This allows build up of permanent (until used) "fuel stocks" through successful missions or challenges. Then a weekly "marathon session" using stockpiled fuel and "contracts" to boost in-game cash rewards. Cash rewards are spent on new planes and/or equipment and weapons systems to upgrade the aircraft to work your way through the tech tree. There are four major aircraft classes: fighters (strong in air to air combat), multi-role ("equally adept at air to air and air to surface combat); attack (strong in air to surface attacks) and bombers. I tend to stick with two fighters and two multi-role in my hangar and choose the best one for the mission and/or to suit "challenges" that generate cash, fuel or rewards to advance in the tech tree.
ACI is highly addictive. I have found the waiting to find a suitable lobby (or "room") and waiting in the rooms lasts twicew as long as being in-mission so count on up to ~15 minutes per sorties, from woe to go.
You will find the following website immensly useful in explaining the game concepts, aircraft types and missions: http://acecombat.wikia.com/wiki/Ace_Combat_Infinity
The aircraft themselves cover a large selection of actual models dating from the Vietnam era, through the 1980s, 1990s and early 2000s and some experimental and more "gamey" hypothetical models. Each aircraft has its unique stats and capabilities, including specialised weapons, and can be upgraded to suit the player's style (mainly via unlocks on the tech tree).
A full list is here: http://acecombat.wikia.com/wiki/Ace_Combat_Infinity/Aircraft
SCORE: 7.5/10 for pure fun and challenge of the online co-op game.