Just finished playing this with my fiancee.
This is definitely my favourite co-op game I've ever experienced. I was genuinely delighted by the sheer amount of variety in the gameplay and puzzles - every level is drastically different both in functionality and aesthetics, with Cody and May receiving different powers/abilities in each location that do completely different things. Even the fundamentals of how you're playing change sometimes - in one level, you switch from a 3D puzzle-platforming adventure to a top-down battle RPG.
The writing skewed a little juvenile for me - it doesn't come across as 'for kids' or anything at all, it's clearly an adult-oriented game with its themes, but coming at it from the perspective of someone who generally enjoys more serious stories, it wore on me at first. The characters all have moments of insufferability, especially Dr. Hakim (seriously, fuck that book, I hated every time he showed up), and there are some story beats where if you're taking this seriously you're going to have genuine issues (I've seen a lot of people ranting about the Cutie sequence).
So I stopped taking it too seriously. Problem solved. I played it as a mostly fun, light-hearted, mechanically interesting multiplayer experience I could use to bond with my fiancee over, and had a pretty great time with it as a result.
My fiancee did, however, get far more into it on an emotional level than I did and absolutely loved it - she got a little teary at the end - so your mileage may vary.
The level of difficulty in the puzzles is just right for me - it was never frustrating, but there were still points we had to stop and think together. At one point, we got stuck badly enough that we had to break for the night and come back to it later, and I literally woke up in the morning with a flash of inspiration about it and pestered her until we could get back on the game. Moments like that were great.
There wasn't a single level where I had a bad time or just wanted to get it over with. At one point relatively early on, my fiancee said something like, "If there's an obligatory underwater level like in every video game, I'm gonna hate it" - and then there was an underwater level, and we loved it, because they'd done enough to make the gameplay fun and engaging that it wasn't annoying or frustrating at all. The whole singing shtick in the last level was a bit out of left field and silly to me, but I still enjoyed myself and it was a pretty nice way to wrap the story up.
I also loved that you could follow the evolution of Cody and May's dynamic, with them antagonistic and uncooperative at the beginning and then slowly beginning to compliment and thank one another, work together more seamlessly, take each other's feelings into account without prompting, etc. It felt natural and well-paced, which isn't always easy to achieve.
We'll definitely be checking out Split Fiction and A Way Out - maybe the writing in those games will be more up my alley, but either way this was a great start.