Disclaimer: This review is based on the Humble Store version.
140 is a platform game with emphasis on accurate timing and precision.
Game objects move, appear, disappear and change shape to the beat of the music, so you have to pay attention to the audio and visual queues to be able to time your actions correctly and progress in the level. If you fail and get "killed" by barriers, projectiles, falling off a chasm or something, you have to restart the current section.
Levels progress linearly from point A to point Z, with a few checkpoints in between, and a boss at the end. Levels gradually introduce new hazards, challenges and interactions right up till the end, so it never feels like they are rehashing ideas all the time. After completing one level, the next one becomes available.
The game is a lot of fun while it lasts. Everything is very smooth, the music is fun and the visuals, though simplistic, work out very well. It all comes together to create a fun and engaging experience.
The problem? The game is way too short! It only has three levels!
After completing the three levels, you unlock a "hard" mode to keep you busy. In hard mode, besides some cosmetic changes, dying causes you to have to restart the entire level, not just the section.
You may be thinking "well, but at least the levels are long and complex", in which case, I am sorry, but I am going to have to burst your bubble.
Completing the "normal" game at a relaxed pace takes you an afternoon (or about 50 minutes playing non-stop like a madman). Hard mode requires a bit more determination, and it will keep you busy for a while longer, but it kinda blows because it isn't really challenging, just frustrating, because you can lose 5/10/20 minutes of progress just because you had a momentary brain fart.
With no extra content, and considering that levels - boss fights included - behave exactly in the same manner in every playthrough, replayability is thus practically non-existent. You play this game once and you are done with it.
All that said, don't be discouraged: the game is worth it if you can get it for real cheap. You'll kill an afternoon and you will have fun doing it.