The short story: If you're new to MG, BUY THIS BOOK. If you want to completely* understand this game, BUY THIS BOOK. If you want to have a book that you can be glad you spent every penny for, BUY THIS BOOK. :D
MGS4 is my first experience with the entire Metal Gear 'saga.' Since MG has such a complicated storyline, my first playthrough involved a whole lot of listening to the cut scenes.
However, despite all that, I was still a bit lost.
Not only does this guide cover 99%* of the game, but it also contains a very readable synopsis of the entire story. (WARNING: Be careful if you read it before you've finished MGS4. You can get away with reading a lot, but ...) This is what really made this book worth it, to me.
Of course, the rest of the book helped a great deal as well, and is what I'll be consulting during my future runs through the game (one of which started the day after I completed MGS4).
*: This guide doesn't cover the passwords that Konami hasn't released yet. Oh well.