I really did learn the basics of Spanish in 3 months with this book. It took me from scratch to post GSCE standard. I joined an intermediate Spanish evening class upon completing it aimed bridging the gap between GSCE standard and A/S standard and found that my understanding of the grammar was well above the class level. I probably had less vocabulary as they had been studying Spanish for at least 3 years but this is so easily remedied. I progressed through A/S and A2 levels at night class attaining very high A grades and I would give a great deal of the credit to this book for giving me such a sound grounding.
The insistence on introducing grammar from the very beginning means you really gain an understanding of the language and can use it creatively to generate your own sentences whereas many other courses focus on providing you with useful words and phrases but if you do not understand how they are formed and why then it is impossible to progress beyond parroting set phrases.
I think this book is best approached in a very pedantic, methodical way: if you actually learn the vocab and grammar in each Unit and complete the exercises you WILL become competent in Spanish, whether it takes you 3 or 6 months.
Some people may find it a little dry but it really does the job and is very cleverly structured so as to build your skills incrementally. I recommend getting your kicks and improving your ear with the excellent free beginners podcasts from the Notes in Spanish website. Another essential for the Spanish learner is 501 Spanish Verbs, - a life-saver.501 Spanish Verbs: 7th Ed W/CD ROM and Audio CD Pkg (501 Verb) (Barron's 501 Spanish Verbs (W/CD))