There are a lot of stories about military training and then going to war, but apart from being an extremely good version of it that shows awareness of how many times this story has been told, this adds its own ideas, strong relationships, a pinch of humor and other emotions as well as a good sense of pacing.
The first sentences "I did two things on my seventy-fifth birthday. I visited my wife's grave. Then I joined the army." gives you a perfect insight into what the book is like and whether you're going to appreciate it. For me, the novel included everything I want from science fiction. Only nitpick is that these people didn't really read like old folks to me, but that might just be due to this being the future and they're much closer to my upbringing than to that of current pensioners. 
The book reminded me of classic SF that just holds up very well, so if you like the older works Old Man's War can offer you this feeling while also containing what we expect from modern literature.
There are awesome discussions of SF concepts, a good amount of action and everything happening always feels personal to the main character. I just love it.