Haha, it really is incredible - I never thought I'd find some of the best places to eat for less than a fortune in such a small book like that, but apparently I did. It is ridiculously easy to find the kind of place you want to eat (cheapest, closest to you, a certain cuisine etc), and the reviews consist of opening/closing hours, special menus, the highlights of the place, and critique on the food, service and location. It's incredible how accurate they are :) Definitely worth the buy.
If you only ever buy one London guide book, buy this one. London is full of cheap high quality restaurants from every conceivable cuisine. The only problem is it is full of a far greater number of hideous overpriced dives!
This book helps you find the true gems. I have followed its recommendations on many occasions and the advice has been mostly spot on.
My only criticism is their definition of cheap! Is £20 a head cheap?!
Still there are enough truly cheap (5-10 pounds per head) places listed to make this suitable for any budget.