I think this book should be in any toy library, despite the level of experience. Great reference books like this don't come along too often and it's always nice to support authors who are actively involved in the community.
Plus, the forward by Larry Hama is a nice treat and there's a lot of neat character info, mostly taken from some of the proto dossiers, that accompany each character description.
I consider myself pretty experienced and i'm totally glad i bought it. It's a great book.