I love this book! It was my sister and I's favorite book for a year or two. Like you said, I thought the main characters, at any rate, were great, and the plot was also fast-paced and exciting. Nice to see an author who can combine the two!
There are another three books in the series (a prequel and two sequels), but they're not nearly so good. The next two get more into action-novel territory, with less interesting character development and some questionable morals (the second book's all about "We're not going to use the evil methods of the bad guys!", and then they lie and deceive their way to victory...). But still very entertaining.