Out of my whole GBA collection, there is none better than this.
Now that that is out of the way, I will say that I have no prior experiences with Astro Boy. I have no connection to the character or story in anyway. This game still blows me away.
Stages (there are many of them) include beat 'em ups, horizontal shoot 'em ups, and hovering boss battles. While it really doesn't specialize in any three of these, individually they could beat any other game of the same genre. All come with the "EX" gaage. This is a super meter, and when it's full you can unleash one of three very cool supers. One fires a huge laser forward, being the most powerful attack out of the three. The next shows mini guns coming out of Astro Boy's ass, stunning every enemy on screen, becoming the best super when attempting high scores (more on that to come). The last has Astro Boy becoming temporarily invincible, flying high speed through every enemy in his path. Depending on the difficulty level, you can hold from up to 3, 5, or 30+ supers at a time.
Speaking of difficulty, the game is sure to present many tough moments in normal mode, but it's hard mode that can make you pull your hair out. I loved every minute of hard mode and eventually beat it, which was one of the most rewarding moments in gaming history.
The high score system kept me coming back for more, which is very deep but will probably go unnoticed by many, but it is there and it is awesome.
Complaints? Well some say there is too much slowdown, but the only time I really noticed slowdown was when there were too many bullets on screen to count and the slowdown really made dodging these bullets Matrix esque and epic.
Enough rambling, Astro Boy is the best Treasure game to hit the GBA (out of their 3 amazing titles with Gunstar Super Heroes and Guardian Heroes Advance), and one of the best GBA games period. Do yourself a favor and pick this one up.