I loved it. It did have some parts that weren't great, but don't all books?
On the whole, since reading Relentless (and then Severed) I have gone through the entire back catalogue of Simon Kernick's books, and am rapidly becoming a fan.
His books have pace, suspense, twists and are well researched.
I also like the fact that characters from earlier books crop up in the newer ones. They're not in a series per se, but I feel it adds to the realism.