Digimon Battle Spirit was the first of the Digimon games to be realeased for the Game Boy Advance. Originally released for Wonderswan, the screen was a tad smaller. It seems that Nintendo was too lazy to make it larger so they stuck a border on it which becomes increasingly annoying after a while.
You start off with with seven Digimon including Digimon from seasons 1-3. You can unlock secret characters throughout the game inluding BlackAgumon, Agumon Ex, Gabumon Ex, Lopmon, and Impmon.
The game is pretty easy overall as you fight other Digimon and collect little balls called D-Spirits to win. When Calumon comes along, get to him first to digivolve to mega otherwise, run your butt off because there is no other way of defending yourself. Impmon appears somewhere in the middle as a mini-boss and he cheats by sucking all the D-Spirits that have not been collected. Too bad you can't do this when you play as him.
The control is ok and the sound is nothing to go home about. The graphics are... meh. Not worth buying unless you really want it.