I have a Silent Brass system right now as a matter of fact.
Its good when you need to get some late night practice in, but dont over use it. Just from playing with the mute, it creates a lot of resistance (which, depending on how much you use it, can be a good or bad thing). If you use it too much, it can screw up your sound. But if you used it occasionally it will teach you to use your air more effeciently.