Hi,
I have used this textbook.
I can tell you the pros and cons and I suggest you don't read too many reviews on this book because the company have a lot of their staff writing positive reviews for them on all kinds of blogs/forums.
The good aspect of this book is that it will kind of give you a summary of everything you need to know for section 3. If you buy this book, I would reccomend you approach S3 study in the following way. Read through the S3 of this book and when you don't understand something, make a note of it. Keep going until you reach the end of their S3 guide and you should have a list of everything you need to study up. Then use google/wikipedia to understand these concepts (as their explanations are far superior to that offered in this text). This book is good because it gives a brief outline of everything you need to know, sort of like a review.
For S1, I still maintain the best study is just to read a good deal of literature. For S2, I found the sample essays they provided and the explanations to be quite good. For what it's worth I scored 90%ile+ in the GAMSAT but don't want to do med myself, was just keeping options open.
Now this is going to sound like a massive sales plug and I feel shitty writing it. But the only good aspect of this book is that if you can read s3 and understand everything you can be confident you are well prepared in the background science knowledge and reading the essays for S2. So, it's good if you can get it cheaply, which it just so happens I do. I'll sell it to you for $50 if you shoot me a PM. If you already have the Des O'Neill package, however, you are wasting your time buying this book.
The cons of this book include that I found their S1 to be useless and the science explanations themesleves are ridiculous to learn from, they're just a good measure if you understand everything.
Once again, I feel shit that this sounds like I'm trying to sell you the book, to be honest I dont' really care if you buy it or not, I'll just chuck it on ebay anyway, but my review was honest.