The First Tree only lasts about 2 hours and is essentially a walking sim through a handful of gorgeous settings with bits of narrative threaded throughout. That being said, the ending truly is something special, and if you choose to get this game in the future, I'd recommend enduring through the rough narration and reaching the finale. The finale is worth it.
The only shortcoming? You can wipe out the whole campaign in about five or so hours, and while there's plenty of extra content to take care of afterward (a "good" ending, for example, should you nail all the requirements), not much is there to revisit after game completion. But, while it's a short journey, it's one very worth playing. Oh, and did I mention the game is littered with pop culture (especially video game) references? I won't spoil them for you, but never thought I'd see a Super Mario RPG reference ever again.
There are secrets to be found throughout the journey in order to unlock an alternate ending, so don't be afraid to fight the constant urge to move right toward the end. Sometimes, it's wise to try going left. Every so often you come across a game worthy of adding to the endless argument, “Are video games art?” INSIDE is one such game.
Ultimately, Bridge Constructor Portal feels like a solid little collection of architectural challenges that will keep puzzle-hungry physics-lovers with plenty to do. But, anyone looking for a heavy dose of story or a treasure trove of Portal-related content shouldn't expect much.
The story elements that are fleshed out don't really amount to much. It would have been better served without the bloat of the story. It harkens back to the oldest of game truths; as gamers, we ultimately don't care how or why Bowser kidnapped Peach (possibly something cake-related?), we only care how we're going to save her, and the playable journey ahead.