I just finished reading this book this past weekend, and was checking to find information about the sequels (which shows that I liked it at least somewhat); I looked for them at the local library, but all of the many copies were checked out!
Many others have given a basic plot summary, so I won't bother. I liked many of the characters. Acorna in particular seemed rather young, naive, but with a huge heart that was willing to do whatever necessary to help those in need (even at the risk of her own life and well-being). This seems a lot like what I'd expect if a unicorn (at least one that fits several of our stereotypes about them) became a human being. I also liked the fact that many of the other characters seemed quite human, and the interesting point that the authors made that people may be neither good nor bad. For example, one of the main characters was an extremely rich old man who was extremely corrupt, but in a good way; i.e., he would bribe officials, pull whatever strings he had, etc., but all for the purpose of trying to save children from slavery.
On the other hand, the story was somewhat predictable, and seemed a bit too easy. I won't say what happened at the end, but even though the authors set it up well, I had the feeling that it had all been sewn up a bit too neatly. Besides this, some of the plot twists were almost too simplistic.
If you can make it beyond that, the end was the kind of ending that you really want to have with such a situation, and I have to admit that a part of me would have been disappointed had it turned out otherwise. One final saving grace is that the conclusion didn't sew up all the romantic situations; I get quite sick of books where everyone automatically and unexplainably end up with the right person after a few quick pages. So overall, I'd call this a decent book; not wonderful, but not a bad book, either.