It's pretty sad when one of the best Gundam games is 6 years old.
The game's plot isn't something akin to the more recent Dynasty Warriors: Gundam. Instead, you just play through condensed (and sometimes extremely wrong or modified) versions of three timelines: Universal Century (the original Mobile Suit Gundam), After Colony (Gundam Wing and Endless Waltz), and Future Century (G Gundam). The storylines (while they vary for the 8 playable characters) are usually faithful to the actual anime, but the lack of a few key characters does make it hard for most fanboys to relieve their favorite Gundam stories.
Gameplay is like your standard fighting game. You've got the big, slow guys. The medium, well-balanced fighters. And of course, the small and weak, but fast underdogs. And of course, severe unbalance when exploiting certain moves of a Mobile Suit (Epyon comes to mind) or playing as a boss (Psycho Gundam is a freak of nature!). Like any other fighter, there are combos, special moves (ranging from standard gunfire to Hadouken-esque fireballs), unblockables, and (of course) supers. In addition, characters can also dodge attacks by "stepping" into the background, create energy shields, and fly. At first glance, BA2 might look like it's easy to master, but the more you play, the deeper and more technical the fighting system becomes.
The graphics are a mix of 2D characters with 3D backgrounds. The 3D could have been cleaned up a bit, but the game still is pretty nice looking.
The music is a good mix of slow, jazzy beats and fast, rocking tunes, as well as a few epic songs. I'm surprised at how good the musical selection is for this game.
The game also has the luxury of having the actual VAs (or some of them) reprise their roles as their respective characters. Unfortunately, most of the characters that are voiced only have about 2 or so lines. They could have at least had more lines so that you won't hear the announcer say the same generic lines ad infinitum.
Battle Assault 2 makes mass improvements over its predecessors (don't pick 'em up) and yet is still better than just about any other Gundam game. If you can find it, pick it up. BA2 has a whole lot of replay value and you'll be sure to have fun with it.