So far, it does seem a really nice game, even though having to switch dimensions often is a tiresome duty.
Anyway, I have already play Multiplayer in this game, with my brother (2v2).
And I love it
This system is a little different that Singleplayer, because you can catch power ups like the different weapons, super missiles, etc, but it also added a new beacon (I call it that way because it is similar somehow to StarCraft's beacons), with different kind of upgrades. These are more like powers. There is invicibility, infinite weapon ammo, infinite missile ammo, invisibility, and some other things, like, for example, Death Ball, a power that will kill any enemy if you touch him in Morph Ball form.
--Edit--
Forgot to add, all power ups (including weapons) and the other powers will disappear some time later.