Overall, I enjoyed this book. We get a few interesting new aliens, a nice twist on the possession theme, very few asshole characters, and we get a more plausible Ace back.
Sometimes we get a reminder that Ace was always smart and tough, even in non-military ways, and that she is a good person deep down despite her recent jerkitude. The mini-bombs are cute, her way to get herself out of trouble in the garden was creative, and I deeply enjoyed the way she had to relearn that a gun is not always the solution. The Doctor's trust in her is clear and unquestioned. He also trusts her to keep an eye on him at need. I deeply enjoyed the scene where Ace realized what was going on before the Doctor did.
And yes, the Doctor makes a few mistakes here. His inventiveness and ingenuity get him out of most, of course, but there are casualties. Sadly, it seems the characters blow them off a bit easily. It's one of the book's weaknesses.
Speaking of weaknesses, I mentioned we get very few assholes. This makes the one or two we do get stick out, which is another weakness itself. However, the author is being careful to show us that even bad guys have reasons for what they do. By the end, I found myself thinking that even for Doctor Who, this was incredibly kind. I didn't need a mustache-twirling villain, mind, but at about two-thirds through it got to the point that introducing one would have been not just bad, but painful. I'm glad he didn't go there.