Great controls and single player. Quite replayable. Lots of unlockables and upgrades don't get used by the player.
One player is limited to the single player campaign. Four players can play together in Ad Hoc, and there is no Infrastructure mode. I haven't played Ad Hoc, but I did drink deep of the joys of the single player.
Let's get technical. The game looks great. The terrain is nice-looking, considering that it's huge and open with a lot of elevation variation, such as cliffs and canyons on some levels, and cities (mostly flat with some 3D buildings) on others.
Planes look great. When I see a plane doing a flyby on the screen with only the plane and the sky visible (no terrain or HUD), it looks almost real. Low-memory terrain allows for nice-looking planes.
The audio is good, with no repetitive sounds. Though, because of this lack of repetitiveness, the machine gun is almost completely silent. Maybe jet fighters just have quiet machine guns?
The music is very fitting to the gameplay, whether you're cruising to your objective or dogfighting with enemy planes. The voice acting is great, though it often sounds like one guy is doing a bunch of different characters with the same voice several times in a row. You hear both what your allies and your enemies say, even if your enemies aren't talking to you.
The controls are perfect. You have a number of scheme selections, and though I didn't see the need to, I THINK you can customize the buttons. The two overlaying control schemes are "Normal" and "Novice". Normal is realistic, where you roll left or right and pull up to turn. Novice is more basic, where you simply push the analog stick left or right and the plane will automatically turn and level itself without the need for rolling or adjusting your yaw/pitch.
The single player is fun and pretty replayable. At the beginning of every level, you select your jet fighter and your special weapon. You get a machine gun and missiles by default, and the weapon that you selected beforehand.
You can buy a lot of different jets, jet-specific weapons and jet-specific upgrades as you unlock them by beating levels. Sadly, the planes aren't very well-balanced. The planes go in order from worst to best, getting more expensive as they go. You unlock at least one plane after beating every level, and chances are you'll never use the planes you unlocked except for the really good ones (assuming you unlock a good one ahead of time). You have to pay for the planes after you unlock them, and you can't afford to buy them all, so adding that with the rate at which you unlock them lowers your chances even more of using most of them. So, though there are lots of jets to choose from, you may only end up using a half-dozen out of thirty-or-so to beat the game, if even that many.
Not only do you have to buy jets after you unlock them, but you must also buy new upgrades for them as you unlock the upgrades. Upgrades are very costly, and each upgrade only effects one plane as far as I'm aware, so you'll probably concentrate on beefing up one plane and ignoring the rest.
You can buy two mostly-unique weapons for each plane, but that means that the expensive weapons are plane-specific, so you won't be buying too many weapons, either.
This level of customization is disappointing, as the game has a lot of planes, weapons and upgrades to choose from, but the way they set everything up corners you into using a very small number of planes and rarely buying upgrades out of fear that you might later get a better plane more deserving of investment.
Levels are very open, with objectives all over the place so you can tackle different objectives from different directions. Enemies range from ships, ground vehicles, anti-air enplacements, a super-mega-plane-boss... Y'know, the good stuff!
The player always has at least one flying ally accompanying him throughout every level. Though allies aren't exactly useful, as they don't take down very many enemies (if any at all), they're fun to have along like the teammates in Star Fox where they have some interesting dialogue and make the player feel like he might actually have some backup, no matter how unreliable.
On some levels, allies also include ships and ground forces, though the player usually has to protect them as they fail to defend themselves.
The thing that's kind of annoying about the levels is that almost every level requires the player to defend something (mainly non-flying forces), and these allies rarely seem to actually be fighting back, though it's hard to tell. It'd be nice if more levels were just all-out dog-fighting between the player and his crew against a bunch of baddies. Enemies in the sky don't usually number very high.
The single player has quite an interesting story. The player's character never seems to talk, and he's simply called Griffin 1 or something like that. Occasionally, a movie made up of hand-drawn stills will be accompanied by the well-done voiceover of a reporter for the player's enemy country to show what's going on with the badguys.
Mission briefings preceed every level. A voice describes everything to the player as a map shows allies, enemies and objectives. The briefings feel pretty lengthy, and aren't exactly necessary to know what you're doing in a level, but are cool to watch.
After a few levels, the player can begin choosing the levels he plays like a board game. Depending on which level the player plays, he can have different assetts in battles that are to come. The first time the player gets to choose his path, he can get "intel" on the upcoming boss, ground forces to help fight the boss (sort of), and fewer, weaker enemies in future levels due to the destruction or seizure of their supplies or forces elsewhere. Or, he can fight the boss right away and probably get a beating.
One thing that's interesting is that at the end of every level, it shows an overlay of the map (the blue map you see in the briefings) and it shows the exact flight and movement paths of everything in the level, centering on the player, showing his changes in elevation, rolls and whatnot. Bullets don't appear, but the flying blue arrow representing the player will fly towards a red arrow and suddenly the red arrow will be replaced by a big, red X, representing a kill. It's kind of interesting trying to guess exactly what you were doing in-game and spotting it as it plays out on the map.
One interesting feature that wasn't really properly utilized was replays. When you die, a replay will play a few seconds (at least ten) before your death, showing everything in the actual game rather than on the blue map. The replays come with a lot of cool angles, and though Random should've been the default, you can switch to it really fast to get a fun-to-watch replay as you crash and burn.
Death replays are cool, but replays go wrong when you actually beat a level. The replay will take a random event wherein you killed something, lasting as long as the death replay, if a little longer. It never seems to capture what I would've considered to be my best kill, or a fun-to-watch kill, so I never feel like there's a point in saving replays. Mainly because most of the game boils down to strafing ground targets over and over rather than exciting dogfights, or the strafing is all the replays seem to capture.
Replays are interesting, but they never seem to capture the best moment, so even though you can save them, they don't last very long and rarely contain anything worth saving. They still add a neat feature to the game.
With great controls and a fun, replayable single player, Ace Combat is definitely worth a grab, and is one of the better titles for the PSP. It's disappointing that there's no online multiplayer, but you can still play with your friends (assuming they have PSPs AND Ace Combat).