IMHO, the works endure in part from the depth and timelessness; the depths with which is explores human fundamentals, and the relative lack of science fiction technologies mean it cannot be dated the same way that, say, Asimov's computers or Star Trek's communicators look odd in light of what is everyday today.

I've always loved the fourth book the most, and I've always felt it was most simply described as "Nietzsche's Gay Science, but to a plot." and I will totally talk about this with anyone who wants to.