This was a mind-blowing book for me. Tipler lays out a testable hypothesis (The Omega Point Theory or OPT) about the end of the universe that shows how it would be possible for "life" to last forever in subjective time, even though the universe will end in a finite amount of "proper" time. Furthermore, based on projections of computing power available near the end of the universe, Tipler tries to show that the resurrection (via perfect emulation) of all humans who have ever lived is not only possible, but very likely.
This "Omega Point Theory" comes across as extremely controversial because of Tipler's efforts to integrate religion into his physics. The Omega Point, which is a virtual computer that will have near-infinite computing power, is predicted to have many of the abilities of what most people think of as God. A lot of the book is taken up with passages from religious texts in an attempt to show the the Omega Point corresponds closely to the God referred to in many different religions. There are also discussions of free will, hell and heaven. The section on an Omega Point-created future heaven opens Tipler to easy ridicule by skeptics, since it discusses not only why there will be sex in heaven, but how it will be possible for a man to make love to, not only the most beautiful woman who ever lived, but the most beautiful woman that can theoretically exist!
Tipler takes great pains to make sure that he is presenting a true scientific theory and not just some new age gobbledygook mixed in with particle physics and general relativity. To that end, he makes several testable predictions based on his theory, including a couple that might be resolved soon, the mass of the top quark and the Hubble constant. If recent calculations of the Hubble constant prove to be correct, the Omega Point Theory may have already been proven false.
If you can wade through the math and physics, I highly recommend reading this book. It is a fascinating speculative look at the far flung future.