So bad it's... HORRIBLE!
This GBA version is very different from the others, as it is a side-scrolling beat-em-up game. It's so stupid and easy it's boring. First, it took me 2 hours and a half to complete the game (including the completion of S.H.I.E.L.D. simulator). The enemies are exactly the same from the first level to the last and their attacks do not vary a bit.
In fact, when I tried the game, I knew I was going to play something bad. The only problem is I thought I would be playing a "so bad it's good" game, while I played a "so bad it's horrible" one. Levels are shorter than short. You do not understand the game's plot as it's watered down to the lowest. I guess Activision really wanted to make some money when they decided to make this game, as it would appeal to younger gamers who possessed either a GBA or a DS. By the way, a DS version was supposed to be released, but it didn't.
This game is really bad and you'll almost get no fun from it. It seems it was intended to be made for kids (who were BY FAR the biggest users of portable consoles, before the arrival of Sony PSP and Nintendo DS, which enlarged the portable console market to a broader audience). Just compare this version to the PSP one, and there's a world of difference.
I recommend this game on all consoles, except GBA, where it's a totally different experience, and not a fun one.