If you have never read Heinlein's Starship Troopers then Old Man's War is probably a 3.5/5. Without that familiarity, Old Man's War is an extremely entertaining and off-beat sci-fi novel with the quirky premise of a seventy-five year old man signing up for a new lease on life with the Colonial Defense Force. If you're familiar with Starship Troopers, the novel rather than the movie, then it becomes gut-bustingly hilarious.
Basically, Starship Troopers in it's original form is a coming of age drama about how the military life turns a somewhat spoiled and aimless young man into a hardened veteran. It's the classic story of the military sorting him out. It also talks about the benefits of a limited democracy where voting citizens have to serve in either the military or some other social service. Oh and there's the mech suits fighting against the Bugs that are the genocidal inspiration for Zerg, Tyranids, and creatures from Alien.
The first half of the book is easily the best with complicated world-building, interesting characters, and unusual development. The premise of the world is humanity has managed to reach the stars but what they've discovered is a reality where the majority of alien races are relentlessly hostile. In order to fight these opponents, humanity has developed specialized technology that allows consciousness to be transferred to young transhumanist (and green skinned) bodies that will fight these aliens on a thousand worlds.
John Perry is a great protagonist as we follow him as he deals with the death of his wife, the fact he has nothing left to look forward to, and he's somewhat estranged from his son. The future of Earth isn't much changed from today (there's a reason for that) so it's easy to empathize with its inhabitants. Watching the universe unfold through his eyes is both a poignant as well as fascinating experience.
Scalzi does an excellent job of justifying each of the elements which makes this bizarre situation. The transhumanist humans are all sterile and chosen from the elderly because the vast majority of them are going to die in battle. The elderly are people who have families and loved ones who they want to protect, which motivates them to want to fight harder while also being expendable since they were going to die anyway. Following the protagonist as he experiences a return to youth and the sudden violence of his new life is all very interesting.
The parody of Starship Troopers is this is a "returning of age" drama which also critically analyzes a lot of the elements of the original book. While the Colonial Defense Force portrays itself in the rousing jingoistic terms of the Federation as well as almost all aliens as mindlessly evil, the truth is far more complicated and horrifying. Paul Verhoven touched upon some of these themes in his controversial adaptation of the novel but Scalzi does it with a more deft hand. Even the narrator's "voice" is similar despite one being a teenager and the other being a septuagenarian.
Unfortunately, the second half of the book isn't nearly as original as the first so it starts to resemble other, less original, military science fiction. The love story between our protagonist and a clone of his wife is also creepy rather than endearing. Even so, there's some genuinely crazy and hilarious moments spread throughout. One of them involves the protagonists stomping through a colony of tiny aliens like Godzilla.
In conclusion, I have to say Old Man's War is an entertaining novel. It's not a great novel, though, because it requires a bit more familiarity with Starship Troopers than perhaps necessary and the second half is weaker than the first. Still, I recommend readers give it a try--especially if they are familiar with the original work. It isn't a perfect novel but it's certainly an entertaining one and sometimes that's all you need.