Complete disclosure - I received a reviewer copy in the mail for free from the publishers.
I have been using GIMP for years. For most tasks I use GIMP, but occasionally I use Photoshop for some very special filters it has. My impression is that GIMP's noise reduction (which was called greystoration, but now is called g'mic) is better than PS's. My impression of this book is that it is aimed at beginners. Nevertheless, I like this book for several reasons.
* There are a number of different easy-to-follow examples laid out in detail, which I personally enjoyed reading through.
* I find it a useful reference for a number of little things. Most or all of this type of material is probably on the internet in some form or other, but I like having a physical book where I know I can find what I want. (I guess this makes an old man!)
* I like that GIMP is free and open source, developed by volunteers, and that Packt (I've read) contributes back to the GIMP organization.
On the other hand, the book has some drawpacks. First, the physical book is in black and white. (The pdf and kindle versions are in color, and are cheaper.) Secondly, it seems to me to be a bit over-priced. (Again, maybe this merely shows my age.) However, if you want to learn about GIMP, this book is well-written and worthwhile checking out.