There is little doubt that The Pimsleur CDs for Spanish I will get you off the ground and learning Spanish painlessly (if not exactly rapidly). If the thought of learning Spanish while being stuck in rush hour traffic, or driving across country, is appealing to you, then Pimsleur is the good stuff.
Having said that, I think it pays to be aware of a few things before making the Big Purchase.
Because the price tag for these CDs is high, you might find it helpful, as I did, to try Pimsleur's "Quick and Simple" set of CDs for Spanish first. Those CDs duplicate (with minor differences) the first 8 lessons of the Spanish I collection. So if you wish to be cautious, that is probably the best way to get started with Pimsleur products, rather than buying the more expensive, more comprehensive products right away. You'll get a sense of whether this is the route you want to go if you want to learn more Spanish.
Whether you start with the Spanish I package, or the Quick and Easy CDs, you will be exposed you to the heart of what makes the Pimsleur method unique and painless. The approach is based on (1) The Principle of Anticipation (a mastery technique that is different than rote recall), (2) Graduated Interval Recall (an approach that provides new stuff to memorize at optimal intervals), (3) Core Vocabulary (an efficient, optimal selection of a small number of key words that you need to know, as opposed to an extensive vocabulary), (4) and "Organic Learning" (learn speech, eg sound, rhythm, intonations, as opposed to a bunch of textbook gramatical rules). I should add that there's something subtly humorous about the content of the CDs, so if you have a sarcastic or wicked sense of humor, you won't be totally bored.
Like many, I found the approach efficient and useful, and I had lots of fun with the CDs as I drove around town blathering away in Spanish.
But it is worth emphasizing that Pimsler is not the only game in town. There are other very high-quality competing approaches (e.g. Platiquemos Spanish), that are generally less expensive. On Platequemos, for instance, the speakers sound like they are from Central and South America, and one gets the impression that they are teaching a version of Spanish that is useful in the Americas.
I think it pays to be aware that with Pimsleur you are learning a very general form of Spanish that does not always play well in some Spanish-speaking counries. In the lessons, for instance, you are quickly instructed to say "Encantado" (pleased to meet you) after meeting someone. When I mentioned this to some Mexican friends, they just laughed and said "you probably won't get killed for saying that, but from know on just say 'mucho gusto' and nobody will get hurt!" A little bit of knowledge is a dangerous thing!
The culture-specific meaning of some words is a big issue. Take words like "familia," "confianza," "amigo," "amor," and "tu." We may think we understand the implications of these familiar words, especially after learning a little Spanish, but newcomers are unlikely to understand their deep and subtle meanings within the context of (for example) Mexican culture.
So, I think it pays to learn phrases and code words from a specific culture or country that most strongly interests you (something you'll never get from plodding through the simple and general Pimsleur approach). To this end, there are books that often discuss a specific country's idiomatic expressions or slang. For instance, I've been interested in learning more about Mexico's version of Spanish, as well as its culture. I benefitted greatly from Boye Lafayette De Mente's book, "There's a Word For It In Mexico" (also marketed as "The NTC's Dictionary of Mexican Cultural Code Words"). In the case of Mexican Spanish and culture, Octavio Paz' "The Labyrinth of Solitude" is excellent, too. Travel guides for specific countries, such as the Insight Guides, often provide a few words of slang that are unique to specific regions.
It pays to have a fluent spanish speaker as a co-worker, friend and/or lover while you are learning from the CDs, as they might, on a good day, rescue you from whatever linguistic cliff Pimsleur sends you over. If you don't yet have a Spanish speaking lover, you may need to purchase some additional resourses, e.g. "Wicked Spanish", "The Lover's Dictionary," or "Hot International Phrases" to help you get into/out of trouble. And don't forget to buy every Shakira CD that has ever been recorded!
Don't be fooled by the $$ coupon that comes with the Pimsleur CDs. In my experience, a much better price can be found using Amazon.com.