...alright..fine.
Here's my list...
VERY USEFUL
Falco: basicly...Kirby can shoot 2 in a single SH which makes an exellent offense. As reflector-precaution he can duck following it, and avoid any reflections...while sniping w/ his D.Tilt to finish the approach combo or go into a grab. Using it in the air while recovering takes away his edgeguarding game by alot as the occasional distractions buy you an easier time back to the level.
Bowser: Firebreathe is normally not a great move, but in the hands of a small target versing a huge heavy one, it can potentially be devastating damage wise and work well in mind games such as attacking from the air above..then dropping further away starting the fire earlier. Bowser can't move through it, forcing him to retreat, shield(making him easier to attack, once its smaller) or take some damage trying to get through) Its also a great edgeguarding tool against the koopa because of its lower trajectory angle.
USEFUL
Samus: nothing devastatingly different from Samus, except that it is low enough to hit samus before she can grab onto the ledge if u time it right. She's also more susceptible to be hit by her since she is a larger, floatier character that tends to missle rush from her cancels, which provide a small opportunity to blast through her missle and get her when she isn't expecting it.
Mewtwo: Same rules apply here, except since kirby is so small the shadowball when charging provides a much greater defense than it would for mewtwo. Moreso a fancy means of getting under your oponents skin..but charging it into the edge is a decent defense ready to shoot if the opposite way if he goes over/ends up over.
SITUATIONAL
Pikachu: The jolts are a great annoy tactic defensively, but Pika is...capable of sexing through them. Kirby also lacks the ability to keep up w/ Pika's speed for potential followups from them. The reason this power is worth taking is because an occasional projectile is alot better then having your Inhale, since a good Pika will break out and QuickAttack(through any of your attacks) back up to the level anyway.
Sheik: Needles are a great projectile and kirby's multi floats provide more room for mixing up the charges as feints for the real aerial diagonal launch. Sadly, he lacks the speed though to follow up in even a fraction of the possibilities sheik has from them. They work good as a edgeguard tool, too.
Young Link: You can drop multiple arrows while drifting away or towards using your multi jumps as an annoy tactic, or provide shelter for a landing when you're being edgeguarded. And since you're so floaty you can charge them in the air alot easier. Not a great projectile but something that can be ackward for the oponent.
NOT USEFUL
Everyone Else: Basically..i'm saying having your Inhale on hand as a kamikazee or pseudo kamikazee into survive is well worth more, than what others' powers will give you against the owners. Fireballs are caped. Some are too small like Nayru's Love and the swords, and some are just not useful against the owners like Fox w/ his speed and a single IceShot vs. 2 characters.