Audiosurf is a very, very unique game and is definitely a landmark in the music/rhythm game genre. Unlike other rhythm games, which limit you to specific in-game songs, Audiosurf is limitless - it allows you to play any - that's right, any - MP3 of your choice, so you can suit your gameplay to your own music tastes. This freedom is rarely found, if at all, in music games, and it has set the stage for other music games in the vein of it, such as Beat Hazard and the more recent Melody's Escape. In my opinion, Audiosurf has infinite replay value, as long as you have a large amount of songs to play.
Basically, Audiosurf is a game where you can "ride" your music to a track that moves and flows in sync with the song, as you hit blocks along the way. It’s sort of an amalgamation of F-Zero’s gameplay and Rock Band’s note charts (albeit less precise). Each MP3 has leaderboards, and for songs that haven't been played in the game yet, you have the opportunity to make the first play on a song and create the leaderboards for it yourself. Of course, this isn't the case with more popular songs that many people have played. Leaderboards also have a comments section, where you can (obviously) comment on a song. A neat aspect of leaderboards (and probably my favorite) is that if you get dethroned from the global leaderboards on a song, you’ll receive an e-mail by default, though you can turn it off if you’d prefer. You’ll also get an e-mail if an Audiosurf friend beats your score on a song, regardless of whether you have the throne on it or not.
There's also several ways you can play Audiosurf: just for fun or for competition. Audiosurf has a variety of different characters that are well-suited for both beginners and the more advanced players, amongst three difficulties - casual, pro, and elite. For example, Mono is the easiest (but also lowest-scoring) character, with a simple strategy - hit the colored blocks and avoid the greys. Then, there are the more tricky (but higher-scoring) characters along the way, such as Eraser (my most-played character), which allows you to erase blocks and undo erases to create chains, and Pointman, which allows you to scoop up blocks and place them in different spots. It is highly recommended that beginners start with Mono, though, as I have had friends try the more complex characters from the get-go and it did not go very well for them.
However, I’m not going to go as far to say the game is for everyone, as people have different strengths and weaknesses when it comes to video games. Aside from the fact that you obviously have to like some sort of music, quick thinking, reflexes, and sense of rhythm play a big part as well. Also, if you are one of those people that tire out of songs easily (like myself), you shouldn’t have a problem for the most part in Audiosurf, but if you play for competition and someone dethrones you on a song you like, you may end up playing the song repeatedly to get a good-enough run to reclaim the throne. It’s happened to me a time too many :P
All in all, if you’re a music lover and want a new visualization experience to go along with your music, Audiosurf might be for you. Be warned, though - it’s very addicting if you have a large library of music.