The single-player modes of "Normal" and "Classic" sum up to about 150 levels, but not one is as entertaining as five minutes with some friends. Partially because you can gloat when you beat a friend, but partially since the computer is a cunning devil even on lower difficulty levels. Multiplayer is the clear choice for actual fun; proceed past the first few maps of the single player, and the PSP isn't just playing a good game, it's playing a dirty game.
You're much better off playing against friends. The lack of real wireless connectivity and competition is a shame, but game sharing almost makes up for it. Ad-hoc local network play lets you send the game to up to three other players, and all four of you can play on separate PSPs from one UMD. Although the PSP's touchy hardware inflicts a certain looseness on the controls, the heart of frantic and explosive multiplayer is in perfect form here. Being able to actually play with anyone nearby who has a PSP isn't icing on the cake - it is the cake.