Defender for the GBA sucks. That's about the nicest way to describe this atrocity to both handheld and retro gaming alike.
Featuring one of the worst control schemes ever, the classic mode actually manages to break defender. BREAK DEFENDER. That's almost as bad as breaking Tetris, but I assure you, it's happened! Instead of, oh, I don't know…giving the D-pad full control over your ship, the D-pad can only move it up and down, while the shoulders control your boosts. You'll turn it off within 2 minutes of popping it in.
Sadly, the other modes don't fare too well, either. The XG and XGP modes, I couldn't really tell the difference between, or perhaps I just didn't care to. As best I can tell, XG is classic mode with the same crummy controls and slightly (and I do mean slightly) better graphics, and XGP mode is an "overhaul" of sorts that somehow manages to be just as miserable as the other included modes.
Fans of retro gaming and gaming alike should take this as an insult of the personal variety, and not touch it with a 40 foot pole.
-George