The best that I could do when I tried to describe it to my friends a year ago was that it's "entrancing, transcendental, beautiful, and focusing." I still don't have a better way of putting it; Synaesthete combines the rhythm and the shoot-'em-up genres in an interesting way, but there's so much more to it than that. The game encourages you to "trance out" and take all of the elements as one; the music blends with your input while the world transforms because of your actions, and the colors and the quotes about rhythm and harmony and life all come together to make the complete experience something above and far beyond its components. The best I can say is that it's like a bit of a mind-altering drug, in that you feel like you get a little glimpse or taste (or sound) of a higher level of thought and experience. It's really quite amazing.
On the practical side, I enjoyed playing the game on its easiest setting, because trying to dodge around the screen with your left hand while keeping up with the complex rhythm tracks on the right hand becomes very challenging. There's not really a danger of dying on the Trivial setting, which frees you from getting too worked up about getting everything perfect, while still giving you the excitement of trying to keep up with the frenetic pace and feeling like you're doing well and making a difference in each world. This creates the perfect environment to relax a little and take the game as a whole, allowing you to get the complete effect.
Even if the game isn't your thing, though I very much hope it is, I would highly recommend that you download the soundtrack. It's the best techno that I've ever heard; I've been listening to it for the past year and I'm still not tired of it. (In fact, I'm listening to it right now!) has done an incredible job of creating the music that serves as the lifeblood and heartbeat of this game, and it also stands alone as energetic, enjoyable, and eminently repeatable.
So I beg you, Windows users: For the love of joy, please click this.