Armored core's basic gameplay usually bogs down the player in controls or game difficulty that doesnt scale well. Gundam games are awefully implemented as I'm sure even the big fans are aware of. Mechassault 2 is terrible by comparison to this game as it has no mech customization.
This game on the other hand has interesting and fun mechanics. Exellent and even simple controls. Great gameplay that I still go back to when I want a mech game. Soundtrack in game is built in but you can select the tracks you want to play while you battle aka rumbling.
There are 3 basic mech types; light, medium, and heavy. Each with their own parts and equivalents that do not cross over to other types. Each part for every mech can be customized to either save weight, hit harder, carry more weight, have more armor, more slots for expansion items, more ammo, and even rate of fire. If you like armored core for mech types but not the gameplay this game is what you have been looking for.
Downsides are that this has only a few maps to play on at different times of day. Graphics are on par with xbox games of its time. Music can get repetitive if you only like 2 or 3 tracks out of what the game has to offer.