I don't know if Demon Possessed marks the third and final book in an urban fantasy trilogy or if the original three-book arc of the Megan Chase series is a stepping stone for further development of these characters. Honestly, it could go either way. I will say that what exists with Personal Demons, Demon Inside (Megan Chase, Book 2), and Demon Possessed is a satisfying enough (and occasionally frustrating enough) arc of character/story development that it feels complete. I sure wouldn't mind more about Malleus, Maleficarum, and Spud though - I LOVE those demons!
Where the first book, Personal Demons, introduced the characters, world, and mythos, and was a tight, conflict- and plot-driven urban fantasy that deals with Megan's past, and the second, Demon Inside, is a similarly driven urban fantasy that deals with Megan's present, Demon Possessed is only about 55% (the first 50% and the last 5%) is conflict- and plot-driven. The rest is a largely aggravating relationship-driven melodrama that looked to address Megan's future but did nothing to endear Megan Chase to me as a character. I haven't much cared for her since Personal Demons, actually. I barely tolerated her weeping and whining in the second book. I actively loathed her at times in this one and felt she either represented or acted out almost every negative stereotype that exists for a woman in a relationship. As a fan of both urban fantasy and paranormal romance, I have little affection for a character as consumed by her own stubborn humanity and resistance to change in the face of all logical - and emotional - reason to take life by the throat and triumph in the quest for your own life, pleasure, love, and satisfaction. And I detest hypocrisy in all forms, as well as the lack of any ability or interest in compromise. Because of this, Megan really offended not only the kick-ass heroine-lover in me, but the soul-deep romantic, as well. That's a pretty hard thing to overcome. In fact, there were more than a few times in Demon Possessed that I would've been perfectly okay with her timely demise. That's probably not what the author was going for.
All that being said, there were several redemptive aspects of Demon Possessed. While I wish there had been a better mix throughout the whole of the book that presented the balance between the relationship conflicts with Greyson and the external conflicts of someone ELSE trying to kill her (though that theme is getting a wee bit old by now) instead of so much of one being the sole focus of the first half of the book and so much of the other being the sole focus of the second half, overall both aspects really were important to the arc of the series. Combined, they did finally seem to achieve the goal of waking Megan up a bit to the reality of the world around her and the people inhabiting it and forced her to embrace both in a far more proactive, positive manner. I'll never fully understand why it took three books to do so, but that was the author's choice. Personally, I think the series would've been significantly more compelling if that sort of wake up call had happened in the second book and the third gave the reader some time with a stronger, more emotionally secure and powerful heroine...instead of a hypocritical, internal, insecure girl more likely to vomit on your shoes or sob on your shoulder when things got the toughest. But...that's just me.
And while I may have an issue or three with some of the choices Stacia Kane made for the character development of the star of her series, and one or two with the slight feeling of repetition in the formula of each book, I have no issue of her ability to spin an interesting and rich narrative with a complex mythology, a lot of danger, and a threat around every corner. There was nothing at ALL, however, I liked about the relationship conflict in this book. Honestly, it did little but piss me off and make me feel like the author was trying to emotionally manipulate me...that's significant because a good book is supposed to manipulate your emotions - while not making you feel like that's the intent.
Because the relationship between Grey and Megan was finally addressed (I've been not-so-patiently waiting), and because of one particular scene with Malleus, Maleficarum, and Spud that absolutely should NOT be missed, I'd rate this particular book a 3.5 stars...but as a trilogy, I think I'd only rate the entire reading experience as a 3 stars, simply because the story was well told, but not to my personal preference for the darker urban fantasy, and the lead character never really redeemed herself to me, even with the choices and progress made.