This is possibly my favorite movie, and I was shocked to see bad customer reviews of it on Amazon.com. To set matters right: The film is amazing, both as a look at 18th-century attitudes towards music, and as a story about the many different incarnations love takes. The film's sex scenes are probably some of the most beautiful around, and those that feel they are unnecessary to the film are probably looking at the past through puritanical filters. (The twentieth century did not, in fact, invent good sex...)
Castrati were, in fact, very much sex symbols in their time and farinelli, when in the service of the spanish king, was summoned ot him "most nights to sing until one or two o'clock in the morning," interptet it as you will. For more information on Castrati, see "Eunuchs and Castrati, a Cultural History" and also The chapter on castrati in "Singers of Italian Opera".
As far as authenticity is concerned, the film portrays baroque audiences, with theior liveliness and level of involvement, beautifully, and I find the director's portrayal of Farinelli quite satisfactory. The machinery and decadence of the opera of the time is conveyed to perfection, and much research has obviously gone into the film.
Handel's music, of course, speaks for itself. It can be easy to get lost in a Handel opera sometimes, among Da Capo arias, but this movie reminds us that this is, in fact, some of the most beautiful music ever written.