An Introduction
Foucault with his concrete arguments freed sexuality from human established constraints. He set sex free from moral bondage. In our times where people have a fear to speak openly about sex issues especially regarding AIDS, one will find the author's treatise the most relevant concerning "sexual problems", the fear to speak openly about sex.
I also recommend my favorite book about Sex and the Perfect Lover: Tao, Tantra, and the Kama Sutrathis topic

Foucault
Great introduction in the area of sexuality. Can be an asset to refrencing in academic work. In my opinion not really a book you could 'take to bed' as difficult to read.

Somewhat wordy, but deserves consideration
Foucault has been criticized for being too wordy, and to a large extent I agree. He deals with complex topics and histories and tries to mesh philosophy with sexuality with politics with morality, etc. It can be very confusing. But Foucault nontheless presents many unique ideas. He wants you to radically reconsider your definitions of morality and sexuality. The book focuses on the hijacking of, and incessant focus on, the bourgeois-created notion of sexuality.

Sexuality, Foucault argues, is a recently constructed term (17th century-present). It is a term which today conjures up certain notions (which the author deconstructs), and this has been accomplished via the "ethics" of the (European) Christian ruling class. Simply put: it is morality foisted upon the masses. That is his thesis. Strange, radical, unique, philosophical, wordy, but regardless, an interesting read. If you can get through it, it will make you think.

Hard...but worth it.
Foucault is one of the most important thinkers of our time. He is a historian, a cultural theorist, and a philosopher. When looking at the History of Sexuality Foucault does not see powerful figures repressing sex, but actually encouraging people to discuss it. This discourse was encouraged so that sex could be controlled and this discourse actually created what is today called sexuality--a norm that we believe to be culturally independent or universal. The belief that sex is repressed is only another strategy formed through a series of power relationships that desires for people to keep discussing sex in order that this "sex" can be classified and controled. For example: Encouraging a discourse on the act of sodomy enabled a catagory of homosexual to be created. Instead of sodomy being a act that a person may engage in, that person instantly became a homosexual, his sexuality constituting his entire being--how he/she should talk, act, and live in general. The discourse that was encourage to develop around sex enable power to classify and control sexuality--power actually created what we believe to be the "real sexuality". Foucault explains the complicated relationship between power and discourse that developed a set of complicated and sometimes contradicting--and always changing--ideas about what sex is and how we are to approach it.
This book is not easy. I will have to read it again. However, I believe that this book is a good intro to Foucault's very important theories on power relationships. An important factor to be recognized is that this book is a translation from french and, as many people have already expressed, has made it more difficult to comprehend. I did not understand everything in totality but I feel that the most imporant concepts were revealed. If you get confused take a deep breath and reread the previous paragraph, doing this helped alot and gives your brain a second chance to wrap itself around the really difficult parts. This is a very rewarding book that will give you valuable tools for confronting and interpreting the ideologies and power relationships we are confronted with. Good Luck!

Influential and important work, absolutely dreadful translation
I would concur with the Marquis point regarding the quality of the translation, which is obfuscating at best, and downright misleading at its worst. For those with the French, go with the original text (French title "La Volente de Savoir"). But I thought it worth mentioning that there does apparently exist an alternative translation of the work by a Robert Hurley, which has been published rather recently under the title "The History of Sexuality: the Will to Knowledge" (ISBN: 0140268685). Unfortunately I haven't had an opportunity to check out the new translation, though I would love to know whether it's any better.

Incidentally, one aspect of this work which appears to have been only eluded to by other authors, is that as the introductory volume of what was intended to be a more far reaching study, there is a significant portion of the work relevant for those interested in Foucault's (contra Dmitry) genealogical method, which made quite a splash in contemporary political theory, as well as the exposition of Foucault's rather novel theory of power. Unfortunately much is left out, and I would therefore suggest inquisitive readers to acquire the collection of Foucault's essays published under the English title "Power: Essential Works of Foucault, 1954-1984" which contains many texts particularly relevant to this work.