I expected a typical war-plus-science fiction book, but Forever War certainly surpassed my expectations. 
 Even though war is one of the constants in the story, you see it in a different perspective. I really liked the fact that the story doesn't focus only on technology, or tactics, so you get to view war in a less "glamorous"/more realistic way. The author explores the real nature of war, its paradoxes, its futility, its horrors and its costs (specially its toll on people). You end up discovering (if you haven't already) that there's no glory in war, only destruction and pain. 
 When I read it, I didn't know the author, Joe Haldeman, had served in Vietnam, but that explains why it felt so real. He knows what he's writing about. And thanks to his experiences, he's able to depict the effects of war so vividly, that you can experience the same things the characters are. All the training, the harsh environments, the boredom of waiting, the fears, the frustrations, the loss of lives... 
 Since we're talking about a war, there are a few battles and they're well portrayed, but is what I mentioned before what makes it nearly timeless and still relevant... I mean, it's remarkable and at the same time quite depressing, that even after forty years you can still relate its message to any discussion about any war. 
 Another thing that I liked about it is that there's no tedious world-building and, just from the beginning, you, along with the main character, are dropped into the action and forced to participate in a war that neither of you knows much about. 
 By the way, I loved how even the first paragraph hints us of something "out of the ordinary": 
 "'Tonight we're going to show you eight silent ways to kill a man.' The guy who said that was a sergeant who didn't look five years older than me. So if he'd ever killed a man in combat, silently or otherwise, he'd done it as an infant." 
 (view spoiler)[That something being the time dilation and its effects. I know it has been explored in other books too, but here it helped to make evident the changes on Earth and the evolution of society. Culture shock and isolation from the rest of humanity is what the soldiers faced when they returned to a place that was no longer the 'home' they remembered. (hide spoiler)] 
 There's only one thing that stopped me from giving this 5 stars: I didn't buy the whole Mandella and Potter thing... for me it was the least believable part of the book. 
 Nonetheless, it's a great read! Very well written, with a very powerful and thought-provoking story that stays with you after you've finished reading it. 
 I highly recommend it!