I'm using it as time permits. My advice:
Be sure to read the instructions (body posture, have water handy, don't tire your voice).
Do each lesson set until you are confident you can repeat each exercise on demand, without searching for the muscles used. (Remember, the instructions say "Do exercises 1-11 until proficient, then do 12-24 (I think, I don't have my booklet with me), etc"
Don't skip ahead and try to do more advanced lessons...they build on each other.
Record and play back your voice for all audio sessions...listen for what she's emphasizing (falsetto, "Mickey Mouse" voice, etc)
Practice, practice, practice. Have fun with it...this isn't a physics or chemistry class, it's more like a foreign language, or music...If you play with the ideas and lessons it'll be much more effective imho.