Harry has a foul mouth (and so do some of the other characters) sporadically throughout the books, but that aspect is worst in book 2, Fool Moon. The first time I tried to read the series, I stopped in the middle of that one because I was sick of all the f-bombs. There aren't as many in the other books, but they're still there. Violence... book 2 was probably the worst for that too; there's a total bloody massacre in that book. As for sex, there is one very explicit scene in book 5, Death Masks, that I heard about from a review and purposely skipped. Other than that scene, it's mostly just Harry having occasionally lustful thoughts about beautiful women, in terms of sexuality.
So, yeah... They're definitely aimed toward an adult audience.