ITSP is a very concise game. It might not be long, but it's always moving; you're always getting new equipment and doing new things. You're always solving puzzles that are mechanically different from the last. It is nearly devoid of filler unless you actually want to get all of the concept art unlocks, and this is a path that I prefer developers take. I want short games that don't endlessly repeat over long ones that feel like a chore.
The puzzles aren't the most difficult out there, but they're imaginative and are very appealing in visuals/effects. The entire game, in fact, is one of the most visually appealing games I've ever played. The world itself is alive and squirming around you. The animation and kinematics are extremely smooth. The camera fluidly zooms in and out to frame the environment around you; for boss battles this means you are a very tiny ship in front of a monster taking up half of your screen. Areas have distinct color palettes with parallax backgrounds (also with animated squirmy things moving around in the distance), but the palettes are always limited in nature and most feature the distinct black silhouetted foreground that probably caught your eye to begin with.
I have a few dislikes about the game. The first is that it feels like it straddles the border between explicitly telling a story and leaving the story to the gameplay. It has these confusing cutscenes that don't really assemble into much of anything I care about. Honestly, I am fine if you just show me the planet/sun being corrupted by a dark force, allow me to infer that I need to stop it, and leave me be for the rest of the game; let the gameplay tell the story. This plot is not on the scale of Mass Effect or The Witcher where you'd need anything else explained in any other way.
Secondly, despite the quasi-storytelling, it doesn't properly build up to an ending; there is nearly nothing to suggest that you're nearing your ultimate goal of cleansing the planet and no real signifier that the last boss is... well, the last boss. Yes, it's really big, but so are the other bosses, it doesn't strike you as particularly end-guy'ish, so you beat it, pat yourself on the back, and then realize the game is over. For how fascinating the game was and how often it "ups itself" with something even more neat, I was hoping for an ending a bit more dramatic/climactic, something like an end-guy with a mind-exploding size that you have to beat in pieces at a time, or the throbbing core of the planet itself.
Despite a somewhat deflating end, ITSP is fascinating and entertaining. It's a game that makes you laugh because of how awesome its bits are, be those items you collected, the visuals of an environment you just entered, the very clever effects and how they integrate with the puzzles you are solving, or just how big everything around you is. It packs a lot into the several hours it takes to beat and makes them very worthwhile. If the main flaws above were fixed I feel like it could be one of my top favorite games - the potential is there but it got a bit muddled up - but the game as it turned out must settle for "just" being really good.