Classical sudoku is a free game where you can easy start to play on one of three levels: easy, medium and high. And there are a few addition features that makes game more interesting. When user have solved the game he get points (count is dependent from level) that are appear in rating table where user can input username.
User interface is simple, intuitive and have a nice simple look. Also users can play as many times as they can, because program generate games that not repeat.
It's for users which like to play classical sudoku or like logical games. Also it's for user that just want start to play quickly without any additionals features.