This book is fine. It falls victim to the same problem (almost) every small groups book I've ever read does: it claims to be the answer. Allen certainly has years of experience, and a resume that defends his books superiority, but it is often unhelpful to a small groups pastor like myself where I am not interested in overhauling my system.
That said, he offers some very good advice:
1. Don't be afraid of messy. Low control, high chaos systems will allow more people to be involved in groups than you've ever thought possible
2. When people don't say "yes" to something you're offering, find a different thing to offer them
3. Training that is centralized, and early will not help a leader. The best training for anyone is training that comes when it is needed.
4. Don't aim for a number of people in groups, or even a number of groups. Aim for a percentage of your church leading groups--the first benchmark is 16%.
I recommend this book, but it was not as helpful as I would have liked.