Had this for a while, back when it was on PS3 with no signs of ever coming out in English. I started playing it again recently.

There's an 'Official Mode', which goes through the core story of the Universal Century. You play through Amuro, Kamille and Judau's story through a sequence of missions, and once you beat those you unlock alternate missions for Char, Scirocco and Haman Karn.

Then there's the Original mode, which includes all the characters from Official mode and a few of the other heroes from non-UC Gundam series. I haven't played through too much of this so I'm not sure if each character's story is different, but the way Original Mode missions work is that it mixes all the different series together so that they are either fighting with or against each other. The little cutscenes are pretty cool, like one with the RX-78 Gundam facing off against the Epyon. It's all fanservice.

While you play, the pilots gain experience in different areas, as well as the mobile suit they use. The mobile suits only gain levels in reference to the pilot, however, so if you get to level 10 with the RX-78 using Amuro, the mech will go only be level 1 if you decide to use it with Heero.

There is some customization beyond choosing your mech and character as well. During the progress of the game, you unlock certain traits for the pilot which augment his abilities. Additionally, whenever you beat 'Ace' mechs (usually major mechs from the show, different from the usual Field commanders) you can pick up parts from them. These alter the abilities of the mech itself.

In all, it's pretty decent. The levels can stretch on for a long time, especially if you don't want someone to die who is all the way across the map (because more often than not, it ends in a game over). To be honest, if this didn't have the Gundam license on it, I wouldn't care about the game and probably would not enjoy it. But as a piece of fanservice (as most Gundam games are), it's excellent.