It's interesting to think that this started off as merely a splatbook for Dark Heresy, and funny to see just how much this whole great idea ran away from them. Short version: good book, good game, worth the money.
Being more-or-less the same system as DH it is easy for a 40kRPG veteran to get into, but there are some interesting changes to pay attention to. Righteous Fury has been streamlined a bit, for instance. Probably the most interesting change is 'leveling up'. In previous 40kRPGs what class you were determined what skills and talents you could learn and how much they cost, in addition to the XP cost of improving attributes. In Only War there is something called the 'Aptitude' system. For instance, one might have an Aptitude in both Ballistic Skill and in, say, Offense (I'm almost certainly paraphrasing). That's two aptitudes that run along the same track, so talents and skills along the lines of Two Weapon Wielding (Ballistic) will be at their cheapest (the 100 XP track). A single aptitude gives you slightly less of a discount (the 250 track) and having none of a talent, skill, or attribute's associated Aptitudes means you'd be paying full price (the 500 xp track). A character gains Aptitudes depending upon what specialty/class they choose, with additional ones sometimes being granted by your regiment. While this certainly makes a given character specialty lean towards taking certain things it does not actually restrain them from going for the most expensive options; there are no character paths to progress by, merely a small block of 'recommended' advances. While going outside of your Aptitudes is expensive, actual prohibited advances are few to none.
The regimental set-up is very interesting, especially the regimental creation rules which allow the entire party to build the regiment that they come from using a point buy system. This also simplifies the character creation process, as your regiment plays a large part in determining your basic kit, a few starting talents or skills, and maybe an aptitude. Replacing a dead character is much easier, as you are handed a bunch of equipment and goodies at character creation and do not have to worry about being TOO far behind the older, surviving characters.
The Logistics set-up is interesting, in-character for the universe, and potentially hilarious (were supposed to be issued a tank, were instead issued several crates of booze, for instance).
The Comrade system allows for PC created and controlled NPCs to fill out the squad (and for the GM to kill mercilessly whenever the players make a mistake).
The vehicles are a great addition, ESPECIALLY the new tables for Critical Damage as done to vehicles. The Crit tables in the FF 40kRPGs were always a blast, and now we have an entirely new set.
I will say, however, that there was a severe oversight in the vehicle section: no aircraft. The explanation given is that they are more the purview of the Imperial Navy and will thus not be in Only War. But there is an entire type of regiment designed to be jumping out of aircraft, a specialty that is built to handle vehicles and would need just one more option to choose to be able to fly them, and there are pictures of Valkyries. All. Throughout. The Book. Teasing you. And Valkyries had to be flown during the Free RPG Day last year! They clearly are being used, so where are the stats guys? They've used a decent in-universe excuse, and I understand the need to hold things back for future books since this IS a business, but this seems a little too forward for me to just nod in an understanding fashion. So, even though I'm probably nitpicking, I'll wave a yellow card and dock a star for Overly Blatant Milking of the Customers.
That's . . . pretty much the only thing about this book that bothered me. I love what they've done with the Imperial Guard, I'm looking forward to the upcoming splatbooks giving us more pre-built regiments, Rough Rider options, and mixed regiment creation, and would highly recommend that any fan of 40k or gritty military sci-fi get this game and try it out.
[4 of 5 Stars!]