I've barely slept since I got it...
Gameplay: The battles are masterfully done, as with the original game. The Bros. Combos from the original game have been replaced with Bros. Items, which I actually like better! Especially the Copy Flower! The dual screens are used in a couple of ways during battle. Characters are displayed mostly on the lower screen. When your cursor is on top of a Bros. Item, a simple explantion of how to use the item is displayed in the upper screen. And the upper screen is used to illustrate tall enemies (Example: A streched up Pirahna Plant) and whenever something (Enemy, item, etc.) goes over the bottom screen, it is displayed on the upper screen. For example, some enemies jump up into the upper screen and then slam down on to the ground, creating tremors. Sometimes, an item will be tossed up into the air (Example: Smash Eggs) and fly onto the other screen. Outside of battle, the gameplay is generally the same as the previous M&L. The action takes place on the bottom screen and the upper screen displays a map of the area. The babies can leave their adult counterparts at any time and continue on their own, exploring areas the elder brothers cannot reach. If the babies leave the screen (Example: Enter a hole in the wll or a warp pipe), they are displayed on the upper screen, leaving the adult Bros. displayed on the lower screen. You can only control one pair at a time. Whenever you want to switch control to the elder Bros., press A or B. To switch back to controlling the babies, press X or Y. If the babies make contact with their adult counterparts, they ride piggy back on the back of their older selves. While walking this way, the future Bros. can toss their baby selves into the air to reach higher item blocks and switches.
They can also hurl the babies on to high up ledges which cannot be accessed otherwise. After a while, the babies aquire hammers and can use them to drill into the ground and smash things. Later on in the game, the adults gain the ability to do a spin jump similar to that of the original game. They also gain the ability to to roll in to a ball to travel faster. If the adults spin jump into the babies, the babies will launch into the air and twirl around while slowly descending towards the ground. While the babies are in this state, you can control them and get them to places previously out of reach. If the adults run over the babies while in ball form, the babies get flattened in to the size of pancakes. While in this state, they can squeeze under low clearences. So far, I have only seen the touch screen used once, but I won't say how in case of spoilers.
Graphics: The graphics aren't much better than it's GBA predeccessor, but they don't need to be. The game wouldn't work right if it were 3-D. The DS's power allows for many characters to be on the screen at once. Other than that, the game isn't that much different than Superstar Saga graphically.
Sound: Very clear sound. I can't think of anything else to say to elaborate. All I can say is the sound is amazing.
Value: I payed $48.99Cdn for this game and have not once regretted it. I think I will be playing this game for a long time.
PS: The time period for the learning curve may change depending on whether or not you played the original.