I bought Mounce's book as a complete beginner with the aim of learning the basics of Greek on my own. I found it was exactly the sort of book I needed and I was able to get through the book very quickly. It sets things out very clearly and focuses on the patterns within Greek rather than rote memorization, which is a big help. One of the best features is the interactive CD which provides some really useful resouces.
I think the reason some people don't get on with this book is that it is still structured in quite a formal way and uses some academic language. This means the book is very rigorous, but if you're the sort of person who likes to learn little bits from different areas of the language at any one time then the approach may be frustrating. If you identify with that, I would recommend Dobson's "Learn New Testament Greek", an excellent book that takes a very different approach.
I suppose it comes down to knowing how you prefer to learn. My opinion is that if you like to have everything presented to you at once in a rigorous way, this book is the best available, but if not perhaps you'd be best looking elsewhere.