Cuphead took me a little bit longer to finish than it did for a lot of folks (I clocked in at around 15 hours for my full playthrough). I didn't sit down and play it in long stretches. I did a couple of bosses at a time and then set the game down for the day. This is NOT to say that the game isn't supremely well crafted, and well deserving of longer play stretches. It is, however, a game that works your reflexes quite well, and a few encounters left me breathless and exhausted.
Cuphead is (and this is no small feat) the most enjoyable "challenging" game I've ever played. The much-lauded art style is tremendous, the jazz soundtrack is perfection, and everything just feels tight and responsive as a good side scroller should feel. The challenge never felt annoying or cheap. Every hit I took was a fair hit. Also, as further encouragement, the developers include a little animation in the menu after you die that shows how close you are to the end. Little touches like these mean a lot to gamers, and I'm glad the developers went that extra mile to include them.
My one criticism is that I found a few of the upgrades and specials to be somewhat wasted, while others seemed to suit the task far more often than not (the smoke bomb, for one). Still, this is a minor niggle. Excellent work from the Torontonian developers. This kind of talent makes me proud to be Canadian.