im gonna say it, this game rules and is kinda one of the best tmnt games out there. practically as good as the beloved games of the 80s and 90s.
after a surprisingly lengthy cutscene for the gba, you get dropped into a level without so much as a weapon. the reason for it in-universe is flimsy, but from a gameplay perspective, i get it. you got infinite shuriken though, but they aren't all that useful for taking down enemies. and for the first third or half of most typical levels, you're essentially in full stealth mode (how many tmnt games let you sneak like a ninja?) and it's not the most complex since you don't need to sneak past more than a few enemies before you get your weapons, and sneaking past them is literally just "walk behind them, hit the up button to hide in a doorway or something, wait for them to pass and then move yo ass" but once you do get your weps, it turns into an almost-but-not-quite metroidvania version of the first game. the combat is still simple and satisfying with great pixel art. sometimes the path will be incredibly linear, but other times you get these branching paths that'll either get you a keycard to advance the real path, or a path that only one turtle can access to get collectables (i'll get back to that). each turtle does have their own slightly different moveset but they also have some special mobility that none of their brothers have. raph can climb with his sai, mikey can hover and also throw his chucks to break blocks the others cant reach. donnie can... do a high jump but it only goes up and like a couple pixels to the left or right, and also its not much higher than a regular jump... i still dont know what leo can do... i spent most stages just playing raph or mikey, they can get most the crystals. donnie i think is only good for entering the maybe 3 dark rooms in the game that have their own crystals, but you only need 85/100 from each stage to move on to the next, so i only ever entered a dark room once.
occasionally there will also be a metroid-like escape sequence where you need to get through a level in one minute. the timing can be kind of strict when you need to find "optional" crystals, but since the levels are only a minute long, its not too bad to redo them to get things you missed.
like the first game, the traditional gameplay is also broken up with arcadey vehicle sections. there arent too many of them and they don't last too long, just enough to be a welcome change of pace.
and of course theres the bosses... they're whatever. sometimes they can be annoying, sometimes they can be easy. besides a couple cool designs made for the game, theyre hardly memorable
but my real gripe with the game lies in its collectables. to move on to the next stage, you need 85 crystals out of 100. arcade sections and bosses contain 10 crystals, all other levels in a stage contain 20. since the game is fun, getting through the levels once is fine. but having to get through them again just to look for any collectables you missed, having to once again collect your weapon partway through. it's needless padding and collecting them should not be a requirement.

overall though, i had a good time with the game. it could do with some improvements, of course, but theres nothing in the game to outright ruin it. i can only hope tmnt3 on the ds continues to improve the formula when i get to it