Mechtavious Destroyer, the internet tells me, is one of the main villains from the cartoon. He consists of 4 little biomechanical guys and is definitely not a sphere. The design is a bit...weird. I'm not exactly sure what the guys are supposed to be. Obviously there are restrictions on how things will look when it has to hold into a ball, but these guys don't fold up so the designers could make them look however they liked. And of all the limitless designs they could have chosen, they went with Weird.
Articulation is limited and you can't pose these guys very well. Each of the little chaps retains the 'place a bit of metal on a certain place and something pops out' element from the Bakugan spheres, however what springs out isn't especially clever, which made me a bit sad.
Stand the little 'uns on top of each other and they form Mechtavious Destroyer. The instructions aren't the easiest of things to follow and it's far, far easier to just look at the picture on the box to see how they're supposed to combine. Basically, they stand on top of each other. The faces of the little guys are still visible, MD retains all the little legs of the individual robots, and there's nothing that really meshes the four together into one large creature.
If you asked my boys about Bakugan they wouldn't have a clue what you were talking about. Mention 'Transformer Balls' instead and they'd be covering the radiator in the things from their toy box to show you how they worked. It was only when they saw the metal card in the box that they realised that MD was from 'Transformer Balls', but since MD doesn't really transform they had little interest in it.
If your child watches the cartoon or plays the card game then I'm sure MD will go down well, but as a toy on its own MD isn't that great and doesn't fit particularly well with the spheres/pyramids/cubes etc that have already been released.