Just played through the vanilla game with this. Got a few notes.
Fang is by FAR the most useful boss to use the insta-shield on! The slight increase to range coupled with the invincibility lets you easily attack him when he tries to jump on you, which normally ends badly. The fantastic upwards range also helps.
I find using the insta-shield most useful in Red Volcano Zone, followed by Egg Rock Zone. The enemies and the stage hazards in RVZ have defensives that are perfect for the insta-shield to cut through, and ERZ lasers are also a good opportunity to shield past.
The chance to insta-shield those dumb turtles in Arid Canyon is very satisfying as well.
I do have one question, though. The insta-shield activates just by holding spin and then jumping. Is this intentional? Or is it supposed to be more like a shield, where it only activates once you press spin in the air, and not by holding spin beforehand?