Is Cuphead a must own game?
Yes. I think so.
The only reason I'd steer you away from Cuphead is if you really don't like the Steamboat Willy, 30s-cartoon art style (which is CRAZY good! It seriously looks like a cartoon!), or if you hate run-and-gun/platformer games - so if you've seen any pictures of gameplay, you already know this isn't for you.
Otherwise, it's a must own. The concept is strong and well executed and is bursting with personality - the music, the animation, the colors; all of it is beautiful and harmonious. Technically the is sound, and while it is a tough game, it never feels unfair which speaks to the attention to detail and balance the team has poured into the game.
The only caveat - I'm only 2-3 hours or so into the game, so we will see what the longevity is, but I feel I've gotten my money's worth already; let's see what they add to the cherry on top of the sundae.