In college, I was involved in the Honor's program, where we took entirely seperate courses that had a variety of topics. One of the courses I took was called "The Wisdom of Yoga." At the time, I didn't know it was a religion, but it worked out fine for me, as I was still sorta in the period where I was exploring other religions and such.
I found the book to be quite unreliable, for multiple reasons. The most prominent reasons being that the whole book was about one person talking with god. Now how reliable was that? Compare this to the Bible, where you have God talking, communicating, and interacting with hundreds and thousands of people over quite a long time span, and the two just don't compare. Also, the Living Gita put forth some very odd messages, such as that we should strive for perfection and never show/express/have emotions, good or bad emotions.
The yoga stretches can be good exercises, and I don't have a problem with that. The problem is what the represent. Most of them, from my experience, represent worshiping or submitting yourself to the earth or sun or stars, etc. This is blatently a violation of the first and second commandments.