Solitaire Challenge is basically a race between two random players to finish a game of Solitaire. Each player is given the same set of cards and they draw one at a time. The only thing you see about your opponents progress is his percentage of face down cards exposed. As you win, your ranking goes up and you begin to face higher ranked opponents. It also seems that the difficultly of the sets of cards goes up, making it more difficult to finish the cards. It takes 4-8 minutes to finish usually.
This game is very fun. You need to be really fast once you get up in the 1400 ranks but if you have fast fingers and a fast mind, you will enjoy it a lot. The only downfall is that you cannot play this without 3g or wifi but that should be expected of multiplayer games. I would give this a 4.5/5 stars. I haven't been able to get enough of it lately. It is free so try it out!