A gorgeous, intriguing game about solving puzzles and running away from things, to put it as simply as I can. There's a very strong attention to detail, and the animations and physics are fantastic. It's very atmospheric, intense, and often unsettling.
There are no on-screen indicators or instructions in this game, you just work everything out by yourself, and it's really excellent at giving you subtle hints through the environment to help you find solutions in an intuitive way. The story isn't explicitly told to you, as there are no cutscenes and not a single line of dialogue. You piece things together by observing the world and what's happening around you, and much of it is open to interpretation. The game will leave you with more questions than answers, and to me, the answers aren't really important. I was fascinated by the surreal journey. I can see why others may feel differently though.
The puzzles are repeated a bit too much, but I don't find this as problematic as some of the more critical reviewers do. One puzzle will teach you a certain mechanic, then throughout the game you'll need to do it again a few more times under more confusing conditions, so you'll be familiar with what's happening but there are barriers in your way that complicate things. Many games do this same steady progression of difficulty while maintaining familiarity for the player, and it's a completely valid design choice. I would say though that Inside takes the concept a little bit too far, and some more variety in the puzzles would have been nice. Another thing I should note is that your deaths can be very brutal, and as you're playing a young boy, I'm sure some players will find that too disturbing.
Though it's short enough that you might finish it in one sitting (about 4 hours), the length seems perfect for what it is. It is a little too short for a $20 game though.