First off, I need to qualify that, while I gave this book a 5 star rating, it is 5 stars for a Doctor Who book, not as a work on it's own; I measure the Doctor Who adventures on a scale to themselves. That being said, I enjoyed this adventure immensely. The premise of the Doctor and a Judoon commander working together was novel, and it allowed me to see the Judoon in a new light. There is a very strong message of "don't judge by appearances" running through the story, and had it not been handled as well as it was, it could have been very after-school-special, but it was saved by the Doctor not being one to expound at length on something so very obvious. I understand from other reviews that his other companion, Nikki Jupiter, was intended to be a strong reference to the Veronica Mars universe, but since I've never watched the show it has no real bearing on my interpretation of the character. She was rather typical for a temporary companion: young, spunky and clever; not bad but nothing special. The plotline was strong, with many twists and turns to keep you guessing, even if the biggest secret was rather obvious from the very beginning. This early "reveal" still didn't undermine the story though, as the excitement then came from the "when" and "how" of the other characters' discovery. One of the best aspects of the story was the fact that the aliens were not the menace, as may have been initially suggested by the opening, but rather humans presented the major threat.
Also of interest, from a fangirl point of view, was the author's subtle hint at when this adventure occurred on the Doctor's timeline: he snaps his fingers to open and close the Tardis, which suggests that this is a post-Donna Noble event and near the end of the 10th Doctor's run.
As an audiobook, Judgment of the Judoon was particularly good. The narrator is a regular voice-over actor for the show, so his Judoon voice was spot-on and his Doctor was really excellent as well, catching all the manic cadences of his sentences and mimicking his accent expertly.
Highly recommended to fans
Well-plotted and characterized
Some minor plot weaknesses but nothing major
Inventive use of recurring alien species
Nice inclusion of timeline-specific details
Temporary female companion underwhelming