This is the game that made me fall in love with the Ace Combat series. In its time it achieved greatness.
It has been some time since I played this game actually, so I will be recalling the facts from memory.
Graphics: The graphics for this game were outstanding for its day. In 2001, when it was released, it was the first game to really push the power of the PS2. The detail in the game was first class for the year it came out and they can still somewhat hold today, albeit not by much. The plane detailing was good at the time and the effects didn't really change much until Zero. The ground textures were fuzzy, the after-burners were not very spectacular, and the missile trails weren't all that impressive really. But that's looking at it today. Then, though, these were impressive graphics.
Control: I always felt that some of the planes were actually a bit too touchy. Some of them had handling characteristics that didn't fit the stats of the plane. For instance: an F-16 "Fighting Falcon" is more maneuverable than a F-5 "Tiger" in actuality. The throttle controls were quite responsive so I have no complaint there. Really, other that a few technical flaws, the control was quite nice.
Gameplay: This game played better than any of it predecessors on the PSX. In this regard AC4 was a major step forward for the series. This game is among my list for top replay value because you never really get tired of it. This is amazing for a game of this type.
Sound: The sound for this game is actually quite flat. All of the Guns sound the same as do the planes themselves. The score was actually the best part on the sound aspect. The radio chatter was also quite compelling as you played through the game. I'd have to say that the sound is the second weakest part of the game itself.
Story: The story for this game was recycled for use in AC6. If you look at the missions, there's very little difference between the two. You start off as a pilot taking off from an unnamed carrier. You then go through the steps of reclaiming the mainland of the Usean continent. This game however has more going on after you take back what was taken. In this game you actually attack the Erusian capital of Farbanti. At the end of that mission you get to finally fight the "Yellow" squadron full on. When you finish this map, level 17, you may think that the game is over... But it isn't. After the mission to take Farbanti you receive word that the new Erusian WMD "Megalith" has gone online under the control of renegade Erusian officers. You, plus nine other aircraft (deemed the Mobius Squadron), rush to the scene to find that now instead of one single formation of five "Yellows," that you have to deal with three such formations. A total of fifteen "Yellows." It is against these odds that you must destroy "Megalith" from the inside. This game didn't have the greatest story to it, but it was decent at its time. Namco Bandai should have left it alone instead of remaking it for AC6.
Multiplayer: Hahaha!!! I am invincible! The multiplayer is actually the reason I bought this game. You can take on a single opponent in any plane in the game here. I have never lost a serious match here. It is as it should be, your skill is what matters, not the plane you're in.
Overall: For it's day this game had very few flaws. The control was decent, even with its flaws, the sound did its job, the score was incredible for a game of its type at the time, the story was good for its day, and the multiplayer was awesome. I'd say that this is still among the best in the series. I hold AC5 as the best with 4 coming in second. AC6, with all its flaws, I hold in third simply because of the online multiplayer. But overall, this game is great.