This is the sort of game you can beat in a weekend. Good for renting but probably not buying.
It's fun, cute and imaginative. Like the old Megaman games it seems very hard at first, but once you get a sense of how the enemies move and how best to attack, it's a breeze.
Here's where it gets to be a blast: If you successfully dodge an attack, your enemy gets a target on him. This means if you hit him in slo-mo he'll take tons of damage and go flying off screen. Once you do this, all the other enemies on screen get targets on them. It's tons of fun to be surrounded by enemies, dodge one attack, then send them flying one by one to their dooms until your VU meter runs out.
There are lots of nice touches. In slow motion mode, you automatically dodge attacks matrix-style. If you do get hit, one of the powerups will give you your health back if you "recover" by pressing a button at the right time. Also, missiles & bombs do much more damage to enemies in slow motion.
My only beef is this - I get a little miffed when your character loses all his accumulated powers between a game and its sequel, but I accept that as an element of gameplay. In the Viewtiful Joe series, it happens every stage. You spend the whole level collecting thingies to build up your "VU Meter" (the amount of time you can use your powers before recharging). Then, at the next stage, you're down to the minimum amount again. Grrr.