The following information is what you really need to know about Armored Core 4.
The missions rarely go longer than 3 minutes, which means they are short and intense. In order to beat the game (without beating all the side missions) I completed just over 60 missions.
Any of my friends can tell you how I would geek out about the customization. I'm not just talking about all the different types of weapons (plasma, laser, ammo, energy sword/melee, rockets, guided missiles, bazooka, etc). I'm talking about everything from body parts (legs, arms, head, etc) to generators for energy, to boosters (back, front, main, over, side, etc), and different CPU boards to coordinate movement and targeting! It's crazy, and I'm geeking out about it now. Somebody stop me.
The game can be difficult, and customizing for missions is especially important in the early game. Later on, after you have established yourself and have spent some hard-earned cash on big guns, it becomes less of a necessity for all the missions. Fitting my mech for a mission never became tiresome. The difficulty was never unfair, it was just right.
There are missions to tackle, that move the story or augment it in some way with side missions. There are also simulations where you battle one other mech for practice. You are awarded money for winning, and the defeated mech's schematics so you could build it yourself (assuming you have the scratch). Very satisfying.
If you want a multiplayer mech game, stray from this title. Unless you enjoy deathmatch, there isn't much here for you to enjoy.
The achievements work very well within the game, and I felt appropriately rewarded throughout.
Bottom line: Mech fans beware, buying this game means giving up your weekend.
RSB