It's helped me to think of the x-guard as an advance on the butterfly guard.
I have been using the butterfly guard a lot in my academy. I'm built like MG (5'6, ~155lbs/70kg), so it suits my game. I have nearly exhausted the butterfly sweep that is used in Garcia's system. Everyone in the academy knows that if they let my hooks in, they are swept. So, they fight the hooks for dear life. They'll let me get one hook, but never two. They never let me get double under-hooks either. They all know what's coming.
That's where the x-guard comes in. If you can get one hook, you can establish the x-guard. It's the next step, which is why this book has such a great layout.
The x-guard has quite a natural set-up from the butterfly guard. It's like pulling out a pistol when the assult rifle is out of ammo (that's how I think of it, anyway).
For those deep half guard lovers, x-guard plays well into that, esspecially if you use knees and elbows to brace your opponent. It's something I've been playing around with. Deep half is quite easy to dive in to.
I think this book, and Garcia's system in general, is GREAT for anyone that enjoys the butterfly guard and wants to have other options as well.
Also, if you have shorter legs and struggle to keep people in your closed guard because of it, the butterfly, x-guard, and deep half become your go-to guards for control. If that's you- this is a great book.