I recently purchased it as well. I love it! I have the 40lb, but I'm currently only using 8lbs during my GPP. It's incredible how such a small amount of weight makes such a huge impact on your conditioning workouts.
From what I've read, you should never use more than 10-12% of your weight (added to the Xvest) for any type of cardio or GPP workouts. I can understand why. However, for dips, pull-ups and other bodyweight exercises, or in addition to using weights, you can definitely use more than 10-12%.
I also think that the 40lb Xvest is going to be the best choice for most people.