One of the most immersive games ever, it'll suck you inside its world for a stupendous, story-driven thrill-ride.
When you start the game, there's an obvious cinematic quality to it. There are elements taken straight from movies, as you'll quickly notice playing Indigo Prophecy. This quality goes throughout the whole thing, really giving the impression of "playing a movie" - in fact, in the credits the game claims to be a movie "shot as a game".
The gameplay is surprisingly well-adapted from a PC mouse to the PS2 keyboard: when you do certain actions, you move the analog stick in the same direction as your character. This works very well, and really puts you inside the person you are playing with. There are also the innovative controls during cutscenes: you move both analog sticks to a direction determined by the game, so that your character moves in a similar way (i.e.: both sticks up means jumping); or they even ask you to press the left and right triggers in a certain rhythm. All of this combines with old-school Adventure elements, so you have a game respectful to its roots, but still innovative.
While not very long, Indigo Prophecy makes every moment very intense. Even with some pretty hard sequences near the end (very frustrating, mind you), the challenge level is well-balanced. After you beat the game, there are many extras to see (which are well worth your time).
A real star of the show is the audio. Both music and sound effects combine to form the perfect atmosphere for each moment - and that's more than supported by the fantastic voice-actors, who make each person seem like a real person. While the graphics may not be a technical show-off, they definitely bring the sound together with the visuals to form that perfect atmosphere.
All of these elements provide an Artistic feeling that's rarely seen in games. Basically, it's like this: Indigo Prophecy is a very good movie, with a great storyline; you get to play and interact with that movie, to the point of seeing different endings - it all boils down to the fact that this game is a work of Art.