I've found that it gets you close enough in most cases.
In fact when JC says it's a difficult walk to get to somewhere, you will often be surprised how easy it is, so I think he's made sure that this is a pretty "inclusive" book in terms of it's users.
It's ok to use in the field if you have a bag to put it in - the slip case wipes clean, and the book is sturdy in my experience. It also tells you how to use an os map and compass if you don't already know. =;o)