Wonderful and incredibly soothing game!
It excels at what it sets out to do and doesn't bog down the gameplay with unecessary fluff. This does in no way lead to the game being boring however, as it still introducing mechanics and makes you use the ones already unlocked in new and interesting ways at a perfect pace.
The first playthrough, between experiencing the world and learning the mechanics, is a joy. By the end you'll probably have all the easy achievements done. If you missed some, a second playthrough can remedy that, while learning to speed up your gameplay for the "99min" achievement. Assuming you don't get it on your 2nd playthrough, you definitely will on your 3rd.
After this there's still the "over 9000" achievement left, and this is where my only gripe with the game comes up. It takes 3 full playthroughs and 3/4ths of the 4th one to get this achievement. Like I've previously stated, the first is a joy. The second is still a lot of fun as you race against time and clean up the last few missed achievements. The third, I was already done with every other achievement and the fourth just felt like a waste of time. I understand them wanting it to be the end goal, but even so I feel like this achievement should only take 2 and something playthroughs, while you're still in love with the game.
Each playthrough, as you might have guessed from the "99min" achievement, takes at least 1½h. That's 4½h when you wrap up your 3rd playthrough (assuming you were quick). And then you still have at least 1h in front of you before you get the "over 9000" achievement.
But like I said, that is my only gripe with the entire game, and that in itself should tell you everything you need to know. Is the "over 9000" achievement a reason not to play the game? ABSOLUTELY NOT! It's a wonderful game, well worth the money. Is it gonna be a pain in the ass to 100%? Yes, yes it is. I'd still give the game 2 thumbs up if I could though...