The preface to the book starts out with the lofty and laudable claim that its objective is to provide a book that 1) does not shy away from providing the reasons behind group theoretic methods to analyze the symmetry-related properties of molecules, yet 2) "'can be read in bed without a pencil'" (yes, a quote of a quote, although I'm pulling it from memory).
Unsurprisingly, it falls short of this ambition; in my estimation, it misses both goals by about the same amount. Cotton had only a nodding acquaintance with the notion of "proof", which cuts both ways, both in lack of rigor and lack of clarity.
That said, it's still one of the more lucid midlevel physics or chemistry texts I have encountered. Actually following the book all the way through, even without doing any of the exercises, allowed me to bootstrap my way to a higher level of understanding than I had when I took the associated class 15 years ago and gave me some serious help in sorting out a core issue in the literature I was reading.