I think the most compelling thing about this book is that Haldeman tried to get his science right in telling his story. In fact, it is the key driving point of the book. Time dilation at near light speeds means that travelling at those speeds means that time passes slower relative to people not speeding along at those speeds. This means that if near light speed is possible, then people will be able to time travel to the future, but the past is left behind.
The least compelling thing about the book was the main character. I didn’t really care what happened to him as he stumbled through the war and the centuries. That he ended up alive at the end, so that he would live amongst a more evolved version of humans than he left is one of the more interesting possibilities of the plot, but even that wasn’t developed enough. It was as if Haldeman was so focused on getting the science right as possible that he forgot to make a character to root for.