At the base level Inside is an atmospheric 2d-sidescrolling puzzler. I think Inside is best experienced with very little knowledge about the setting/story, so this review will not contain any details about those things other than my overall opinion on them.
Graphics: The art-style of Inside is relatively minimalistic, and the only primary colors present are different shades of gray/black with brief instances of other colors. Usually, that color palette would not be a good thing, but it fits the depressing setting of Inside very well. This is an example of what I am referring to.
Story/Setting: As I previously stated, I am not going to give any details about the setting/story, just my opinion of it. I think the setting and atmosphere of Inside is great. However, the story, or lack-there-of, I did not think was very good. There zero exposition told directly to the player. You have to interpret everything that is going on and determine what you think is happening yourself. Now, I am not saying that I want a story to be spoon-fed to me, but throughout my time with Inside, I was mostly confused about the "why is this happening" of the story. Once the credits rolled I was left unsatisfied that my question of "why" never got answered.
Gameplay: The gameplay of Inside is very simple. It mostly involves interacting with/moving objects to progress to the next area, with some chase sequences throughout. The game autosaves very frequently which is good because trial-and-error is a large part of quite a few of the puzzles. The puzzles themselves range from very simple, to moderately complex. The most complex puzzle (which coincidentally was my favorite of the game) took me around 10-15 minutes to complete. There are also several hidden areas throughout Inside containing a machine you pull a power core out of. Overall the gameplay/puzzles were fine, nothing great, but not terrible.
Verdict: Inside is a short (~4 hour) experience that excels at creating an impressive atmosphere but falls short in terms of both story and gameplay.