I really love the way the handholds are both your source of climbing higher as well as upgrades. I also think it's really smart that they come back as black handholds that have a soft punishment for grabbing them (that can be come back from).
I didn't beat the narrow chimney part of the game (my strength was pretty high, but could have been higher) but the level design was very visually appealing at that part.
I don't love the limbo zone before restarting again. I think it is useful in teaching the mechanic, but I would have prefered to just start again (after the second slow motion limbo zone).
I also had some quite entertaining glitching out when the physics got overwhelmed :)
Again, the handhold upgrades was an inspired idea.