Unique, exciting and technically impressive.
It is an arcade game in it's soul w/ a deeper story overlayed and done in such a way that does both styles of gaming justice. The majority of levels play like a 2D platform beat-em-up but quite a few see Astro Boy flying horizontally across the scream shooting his laser at rocket ships and missiles. The thing is even the beat-em-up levels maintain an element of the shooter levels as Astro Boy still has use of the laser he uses on the shooter levels and can also zoom across the screen w/ a double tap of the d-pad.
The gameplay is simply excellent here. Astro Boy can punch, kick and use his laser in combos and additionally has a few well thought out special moves that add depth and strategy. Combined w/ an RPG-style upgrade system, this offers gamers something of a choice in playing styles based on how they want to fight. If you want to rush in and throw punches, you can upgrade life and mele attacks. If you are more the type to like to stick and move, avoiding damage, you can upgrade your special moves and laser.
While combat is quite good, the level design is not the strongest point here. A lot of them feel short and thrown together. They work well enough and are rarely frustrating but they do feel a bit like they are just something they threw in so you'd have something to do in between bosses. It's a good thing the bosses are tricky, fun and overall a blast to play because they are certainly the main focus.
Presentation-wise Astro Boy does really well for itself. The GBA has plenty of juice for an accurate depiction of the titular comic book hero, sprites are big, colors are vibrant and the sound is great. I am essentially unfamiliar w/ Astro Boy in non-video game context and I found the story to be engrossing and entertaining. There are multiple endings and the game must be played through at least twice to get the best one. The overall game is pretty short but that is almost a strength here as it keeps it from dragging when you have to play through again to get the rest of the story.
There are not many games action games that play as well as Astro Boy: Omega Factor and even fewer that do so well on the GBA. It looks and feels great and despite some flaws in level design, still feels exciting even on replays. Even today, this game will make it hard to put your GBA down.