This is one of the best games on the GBA. For starters it's a Mario game and most of those are great. Secondly it's an adventure game unlike any other that I have seen on the GBA. You take control of both the brothers in an attempt to save day. This involves wandering around a huge map and using each brother's skills to get past obstacles. These might involve making one brother really small so that he can crawl through a tiny hole to get to a button and let the other brother through; or maybe having one brother jump up onto a high ledge that the other can't reach; or maybe bashing rocks to get to a new passage way. The puzzles generally are not too difficult but some do take a little bit a thought to work them out.
The graphics are great with the simple bright style that we have all come to know and love in Mario games. The sound is great too. There's even speech. The two together make for some comical little animations and sounds.
All in all this is a fantastic game that should last a while, but may not have much replay value.