This is, simply put, one of the best books I have ever read.
It's deceptively well written. Although the prose is fairly straightforward, it seamlessly meshes hard science fiction, space opera, military SF, and what I think of as "soft SF" (science fiction that maintains a focus on the so-called "soft" sciences, like sociology and psychology). What is most impressive about this is that none of the SF sub-genres represented by this story are neglected; Haldeman treats them all equally, handling them with respect even as he transcends them. Even though the story starts in the far-distant future of 1997, this doesn't really serve to date it as it's also a bit of a time travel story - nearly 1200 years pass in the course of the 250 page novel, so one isn't really given much time to dwell on the incongruity of near-lightspeed travel in the late 90's.
Although there's plenty of cool sciencey-type stuff happening here, the focal point of the book is that near-lightspeed travel. This is achieved by special technology built into spacecraft which allows the craft to utilize linked collapsars to provide them momentum. (For those wondering what a collapsar is, my understanding is that any collapsed stellar object, like a white dwarf or a neutron star, could be considered a collapsar, but the inference here is that Haldeman is referring to black holes.) The speed attained by the craft is so close to lightspeed that relativistic time dilation occurs, thus making it so that for every year the main character Mandella (a name with nearly the same letters as Haldeman) spends in a tour of duty in space, he returns to Earth to find that decades have passed. This leads to massive alienation amongst the returned veterans, a clear analogy for the alienation experienced by Haldeman and other Vietnam veterans upon their return to the States. It's fairly obvious that this is a war story told by someone who has actually fought in a war, as it portrays war in all its brutal senselessness and without any of the pat sentimentality and naive sabre-rattling of most other novels about war. There are no vainglorious battlefield heroics here, only dubious combatants coerced into perpetrating barbaric slaughter by a cynical institution which sees the endless war as an economic necessity.
There is too much good stuff in this novel to go over in a short review. Read it. It is not just one of the greatest science fiction stories ever, but one of the great stories of our time.