Excellent for the unproven or uncertain leader. Warren and Holladay have made it very easy for those not comfortable with seeing themselves as "teachers" to nonetheless guide anyone through their excellent participants' guide book. Some of the anecdotes are less than mesmerizing. But overall, they've done their job superbly -- making it possible for lay people (like me) to lead others through the very best, core and crucial doctrines of the Bible.
I am a youth pastor and it is always a challenge in determining what is valuable in the curriculum that is published. Foundations is a great series to use in order to show the foundational principles in Christianity. We are using this series in our Youth Sunday School and it is great. I would highly recommend it to anyone.