I got Hugo's Gaelic in Three Months and started using it about two weeks ago. To explain a bit about the book (which I am about a third of the way through), it starts with basic grammar and ensures a good understanding of the structure of the language before going any further. For any non-Gaelic speakers reading this, Gaelic is a language which is VERY unlike English. It is a Verb Subject Object language, where English is a Subject Verb Object language. This could be quite confusing, but I feel that this book makes it very easy to understand.
The book recommends an hour a day to get through the book in three months, and the lessons are split into smaller topics, which makes it very easy to refer back to a particular part. Therefore, it would also make a good reference tool after everything in the book has been learnt.
I did not buy the cassette pack, as I have easy access to Gaelic media via radio and TV. I would, however, recommend the tapes for anyone who doesn't have access to these or anyone who doesn't already have a Scottish accent. Some of the sounds in Gaelic can be quite difficult to master for a foreigner.
The book itself is a great study aid, as it has questions through every lesson and the answers are at the back. Great to check exactly what you HAVE learnt.
All in all, I would recommend this book, especially for people who have absolutely no previous knowledge of the language (like me).