The title basically says it all. I bought all three books separately, and honestly buying all three separately was worth it. Getting all three books in one collection for half the price is easily worth it (sorry for the sales pitch like tone).
The Grey Knights Omnibus collects the three Grey Knights novels Grey Knights, Dark Adeptus and Hammer of Daemons. The stories follow Justicar Alaric, a member of the special Grey Knights space marine chapter set in the futuristic and gothic Warhammer 40K series. A special branch of the space marines, the Grey Knights have psychic powers, more advanced armor and weapons and are some of the most powerful warriors available to humans in the grim futuristic setting. They specialize in hunting demons who threaten mankind, often the last line of defense against these powerful beings.
Grey Knights introduces the reader to Alaric and the Grey Knights chapter, delving some into their past and what their chapter is about. Alaric is sent on a mission to investigate the return of a demon that had plagued man a thousand years before. Alaric and his contingent are forced into a race against time to find the demon and battle its fodder, both other demons and humans alike, and keep him from returning from the warp to the real world. This is the strongest of the three books, filled with action and intrigue and never a dull moment.
Dark Adeptus revolves around Alaric being sent to a world that mysteriously reappears after centuries from the warp. Landing on the planet Alaric finds it to be infested with the forces of Chaos; the sworn enemy of mankind. This book has a totally different feel to it, with a more exotic environment and the sense that anything can happen at any time, and everyone must be watched. It is not as intriguing or battle heavy as the first novel, but still puts up a good showing.
The final book is Hammer of Daemons. In this Alaric and his men lose a battle and are captured, taken to the Chaos world in the Eye of Terror. Once there he is forced into gladiatorial combat and must devise a way to escape the hellish world. This novel is nothing like Dark Adeptus, so even though the setting on a Chaos planet is a main plot device that is one of the few things they have in common. Much like Grey Knights this novel is filled with action, the most of the three, and has some twists that you won't expect. I should also note there are not many Warhammer 40K novels that follow a path like this one.
Counter's writing is clear and concise, and he wastes no time in getting to his point. He manages to paint a vivid picture of the settings quickly and rarely do any point in these novels does the story drag. If he brings up something you don't know about don't worry; he'll get you all you need to know and you won't feel lost at all.
All in all the Grey Knights series was very good. Of the Warhammer 40K novels I have read Grey Knights is my favorite, and Hammer of Daemons is also near the top. Having all three books collected in one and at a lower price is well worth the buy. You won't regret picking this up, Warhammer 40K fan or not.