Would an HIV-preventing drug change your world?

Depending on who you listen to, the new HIV-prevention drugs (known as PrEP) are either the 'first treatment to reliably prevent HIV' or 'a profit driven sex-toy for rich Westerners'. In other words, the treatment is hugely controversial. But New York and San Francisco are in the process of making it freely available for anyone who wants it and many argue it is the future for the treatment of HIV/Aids. But how reliable is PrEP? And does it encourage responsible sexual behaviour or promote condom-free sex?