Water Shield
Just when you thought it couldn't get better than Jewel Satellite... it did! It's not as good at protecting you, but it's so much more effective at ripping [parasitic bomb] to bits. Castles, soccer machines, flaming birds... they got nothing on this puppy.
Solar Blaze
Good against chill man, and good in some of the challenges where you need to avoid taking damage for a gold crown, but otherwise it's slow and it uses a fair ammount of ammo, so it's not really that great in general.
Chill Spike
It gets an automatic boost for being the weapon that kills the otherwise invincible spine things in this game. And you can have a lot of fun against Nitro Man with it (let him run over the spikes, or even more hilariously, just keep freezing him in the corner), but otherwise it's primarily about freezing things, not about doing damage... which, while useful in some select circumstances, is not a good trait most of the time.
Wheel Cutter
The wall climbing ability is interesting, sure, although I haven't found too many circumstances where it's particularly useful. Still, it's good for dropping on things much like Plug Ball was, so it's decent.
Commando Bomb
Meh. It's good for blowing up those walls, but otherwise... not really. Even Blade Man, who's supposed to be weak to it, is better fought with just the buster.
Triple Blade
Magma Bazooka reloaded! You get plenty of ammo, it's better against any boss than just the buster, and potent mid-stage as well. What's not to like?
Rebound Striker
I don't find myself using it much, save against Sheep Man and the Block Devil. Bankshots never were my style...
Thunder Wool
Blech. Slow, clunky, uses lots of ammo. I'm tempted to call it the most useless weapon in the game, but I suspect Mirror Buster will do it one better on that once the DLC is released.