From what I understand, risky behavior aside, as long as the person sticks to taking it everyday, it lowers the chance of infection by 99%:
Taking the pills is a practice known as pre-exposure prophylaxis, or PrEP, and some researchers believe it may reduce the risk of infection by 99 percent if patients take their medication daily as prescribed.
http://www.nytimes.com/2014/09/18/up...abt=0002&abg=1
Pretty much as safe as a condom. Of course it will not protect you from other STD's, so a condom should still be used, but still, it would help with the infection rate in high risk communities. Personally, I think its a good idea, as long as the person is also responsible.