In reality, Yoga is first and foremost a state of celebration. Yoga is your declaration to the whole world that you are still alive and kicking and that you can do whatever you please with your body in a healthy and productive manner. The most important result of Yoga is that you feel the joy of being alive. This joy comes about as you feel your body work, improve its function and go to places unimaginable in its capabilities.
Historically, Yoga is a form of a body-mind culture, to conquer mortality, along with meditation. It is reputed to endow the practitioner with extraordinary powers, the eight great superhuman capabilities, or Asthama Siddhis--the ability to contract or expand the body, become as light as possible or heavy as possible, have unrestricted access anywhere or obtain anything that one desire, have absolute lordship and control of everything. These capabilities are not meant literally, but figuratively. Everything mentioned here is possible, in your own universe. When you realize this, you are liberated.
I feel that as long as your Yoga practice has made you a happier person, you are successful. As long as you have more and more joy in your life, you are making progress in your Yoga. When you have joy in your life, you spread it around, wherever you are. This joy is infectious and it returns back to you, joyful experiences. Then, you are a true Yogi or Yogini.