Nikki Griffin is a really fun kind of mystery/thriller heroine: she's got a chip on her shoulder, she's got violent tendencies, she's got a murky past, she lives a double life. We get to see her strong sense of morality placed at odds with the law, with other people, and even with her own rage. We're led through a winding mystery, tugged along by Nikki's agency and threats to her life and others'.
The only thing I didn't like was the Sherlockian "I knew who the culprit was all along and I was leading you (the culprit) here for our big confrontation" climax. We, the reader, didn't get the sense of Nikki having figured everything out in advance; indeed, we got to see a steep uptick in her fear. I'd much rather be be paranoid and right by accident than doing a comic book confrontation with the bad guy.
I'll definitely be keeping an eye out for future Nikki Griffin books, though, because I loved her as a character and I think she will continue to be a fun heroine as this series continues.