I don't think Hitstream, or any video game fitness system, should fully replace dedicated exercise sessions outside the house. Keeping things varied is important for staying motivated. I think it would be best to use Hitstream as an addition rather than a replacement for your normal workouts. Plus, I think there's a lot to be said for getting outside and training in nature - my favorite workouts of all are bike rides around the countryside near my house, and that feeling can't be replicated by the gym or a game.