Well, I'm not sure I can finish it. I'm trying to beat what appears to be the last part (don't want to spoil) and I'm not very excited about trying again anytime soon. So I'll lay down my final opinion anyway.
It's a solid design. Innovative in the way it takes the metroidvania gameplay, tone down the exploration aspect and accentuate the action aspect. Instead of calmly navigating through the environment, listening to some soothing piano, I'm always shooting, jumping and running like a maniac.
Aside from a few particular situations, the controls respond well. Shooting in four directions is a feature well explored by the level design; you can frequently find ways to defeat an enemy without necessarily sharing the same tiny platform they stand, shooting up or down (by jumping over it). A few problems: Leaving water with the abilities of wall jump and sink is a nightmare. What aggravates this is that several parts have the water level right in the intersection between two screens, and the transition "forgets" the commands you were sending (if you were moving to the right when changing screens, it forgets so). Also, and this is not a bug, the controls would benefit by being able to control how high you can jump. But the level design takes care to avoid situations where you NEED to jump just a little bit.
As is common in these kind of game, you gain new abilities as you progress. This is nicely done, as each new ability allows you to further explore places you've been before (or not). For example, after being able to sunk in water you try to remember all the places with water you've been through, and they usually reward you well. The abilities are also really clever

In the artifacts part, it could be clearer (hinted in some way) what order would be better, in my opinion.
All right! One cannot talk about this game and not mention the story. Yahtzee's previous games, books, reviews etc are a definite hint that this is his strong point (even though my favorite is Trilby: the art of theft, which is heavily gameplay based). Both the story and story telling are incredibly engaging. Can't say much about it.. it's definitely the strong point of the game. Too bad I didn't get to see the end, yet.
Now to the major flaw of the game, in my opinion: who the hell beta tested this game? D.A.R.Y.L?? The level design undermined with unforeseen dangers that force you to memorize them. I mean, memorize them when the level is kind enough to present you with a pattern, because some parts are filled with unforeseen AND random dangers. So instead of the challenging kind of difficult, it's just plain frustrating. And I say this having played with a very large life meter and the power to exchange mines for life.
At first I thought "hey, this is a cool save system!" But considering the above problem I would prefer something more compact, like a flag you cross or something.
So that's it. Honestly, I think anyone complaining about anything else but the difficulty is just blowing off steam from a recent rage quit. The whole gameplay design is very good and the story - and related aspects - is awesome. The level design needs to be polished though. Too hard and too unforgiving.