According to the ESRB, this game contains: Fantasy Violence, Suggestive Themes
Parents shouldn't be too worried. The outer space activity is limited to shooting enemy ships and flying squids, and the on-foot action is mostly about blasting bug-eyed slimy things, although occasionally there is some extremely simple combat against humans and humanoids. However, none of the violence is graphic and there is no blood or gore. There is no explicit sexual content, although some of the dialogue is slightly suggestive and most of the female characters in the game are clothed in rather revealing bikinis. Still, this is the Game Boy Advance we're talking about here, so the level of detail possible pales compared to a standard home console. There is no use of profanity.
"Shmup" fans will definitely want to check the game out. It's not the best action ever seen, but it is a genuine step forward for the genre and a very interesting blend unlike anything else I can think of. Mixing and matching the gun components was highly enjoyable, and something I'd like to see other shooters adopt. The pace does slow down a bit during the RPG sections, but overall, this is something you'll want in your library.
GBA and SP fans, there's just not much to say. You need to buy this game, and you're going to be sorry if you don't. Save yourself the hassle of buying it for an inflated price on eBay, and avoid the need to kick yourself in the pants later. Just buy it.