I've had this book on my shelf for a great number of years. I'm pretty sure I bought it some time when I was reading Scalzi's Old Man's War, as people made comparisons between the Forever War and Old Man's War, hey, likely because they both had war and distorted times as theme elements.
The Forever War is an author-described commentary on the Vietnam War, where on the lowest level, none of the soldiers know why they are fighting. There was an interstellar ship that disappeared, and boom, now the entire planet is at war with the other race. The other race is somewhat baffling, in that none of their actions are smart in terms of war.
I can understand the comparisons of Old Man's War to the Forever War, even as I think they are both great science fiction books from the perspective (nominally) of the soldier. Haldeman's Mandella is less witty and less self-aware than Scalzi's Perry, likely because the latter is actually an old man, with a lifetime of experience before he entered the military, while Mandella is a two-tour, everyone-f'ing-died, seasoned vet by the time he's 25. Yet, we are still cheering for Mandella as his world just completely messed up.
I haven't and won't read the sequels, Forever Free and Forever Peace, as I feel the story is self-contained and ends well. While I have to say, "Wow, after all the shit that happened in your world, you end up in a sufficiently happy place, life just doesn't work out that way," I'm okay with this book's ending being relatively happy.