Engaging, fun, and just long enough.
One thing that really made the game is the humour provided by the Aperture Computer. Just the credit sequence is enough reward for finishing the game, not to mention the entertaining boss fight. The game isn't particularly hard, but it isn't too easy either. The challenges that you play outside of the campaign add some difficulty to the game and they are entirely optional, so you can choose whether you just want to enjoy the setting, or whether you want to truly play the game.