I've been using this book in the classroom for some time now (7 years) and I feel it is high time to give a review of it. All my comments pertain to Unit 1 only, and the fourth edition of Unit 1 at that. I will review the other units at another time. First and foremost: the little red book.
There are a lot of positive things to say about this book. It is small and rather innocuous in appearance, which is actually a bigger deal than one might think. It does not, as many Latin textbooks do, give the impression that it will overwhelm the user in any way. It rather offers a friendly interaction from the very start - each Stage (chapter) begins with little drawings under which are simple sentences. There are characters (Quintus, Metella, Caecilius, Grumio, and Clemens) that figure into each story. The stories are all intertwined and necessary to each other for overall understanding. Each Stage ends with a culture section filled with pictures and interesting info.
The cons? The grammar set-up takes some getting used to. When I learned Latin (50 billion years ago) I learned all possible information about first and second declension nouns all at once. Did I need all that info? Not really. Cambridge presents cases over an extended period of time, so that when they do appear they do so naturally and in a way that makes good sense. And yet - sometimes I add extra grammar portions to get my classes to where I feel they should be. Another con? The vocabulary index at the back of the book makes me cringe a little. Verbs are presented in the 3rd person singular present active indicative and in the 3rd person singular perfect active indicative only. Weird. I know that the book starts out with 3rd person, but we get to 1st person pretty quickly, and the infinitive should always and forever be offered when giving a Latin definition.
Overall, I am a big fan of Cambridge. They offer a website that accompanies the book for an extremely reasonable fee, and the ancillaries this site provides are absolutely outstanding. I don't use this book for everything in the classroom - I also use TPRS, and I create a good deal of my own worksheets and materials - but I've found the Cambridge Latin Course Unit 1 to be a great building block for my Latin curriculum.