This book has been heavily advertised as the return of the popular X-Wing series. However, it isn't quite a glorious return to the glory days of the 1990s. Rather, it's heavily influenced by what the Star Wars Expanded Universe has become in recent years. It's not all bad, but it's also important to realize before picking up this book.
First, this book relies HEAVILY on recent EU novels. If you haven't kept track of the recent books (and I admit I haven't - I stopped reading after New Jedi Order) some of this stuff might be really confusing. Daala as chief of state, the Imperials still alive and well, a purge of Jedi. Alliston does attempt to bring readers up to speed a bit, but it's a very different galaxy and one that I personally find less interesting and less suited than the previous X-Wing novels set against a still powerful Empire. Also, it's important to know the details of the EU to understand the plot because it focuses on a conspiracy attempt that happened during the post-NJO with constant references to post-NJO events as motivations.
The setting also means that this X-Wing novel doesn't feature Wedge Antilles, the backbone of the previous X-Wing novels, much less the other pilots from the Original Trilogy like Janson and Tycho Celchu. In the original X-Wing novels, Wedge served as the connection to the movies. One of the joys was seeing this popular character in action. In Mercy Kill, by and large it's the next generation of hotshot pilots, including Wedge's daughter. Some are sons or daughters of previous Wraith Squadron members, but given that the last novel came out over a decade ago you'd be forgiven for not even remember who they are. If you don't already know these characters, you might find it hard to really care for them (although there are a few I really like).
If these problems don't bother you much, or if you're steeped in EU lore, Aaron Alliston actually wrote a pretty interesting story. Alliston follows one of the veteran Wraith Squadron members, "Piggy" Voort, as he's pulled in for one more mission. Voort is super-intelligent Gamorrean, but don't let that fool you - he's not in there for comedic relief. Voort has been scarred by the Yuuzhan Vong war and the book actually builds quite an interesting character for him as war continues to haunt him.
The other thing I like about the book is the clever espionage escapades. Wraith Squadron is more than just flying X-Wings and Alliston did a great job coming up with complex and surprising scenarios for the team. The fast pacing and limiting Piggy's knowledge to a "need-to-know" basis makes these scenes fun and suspenseful to read - possibly some of the best Star Wars action I've read in a while. At it's best, it's like reading James Bond or Oceans 11 in space.
Overall, my preference for an X-Wing novel would be something set during the time of the Original Trilogy, where we can see Wedge and the other X-Wing legends take on the Empire. I understand Aaron Alliston himself has written a lot in the FotJ era so this period might be dear to him, but the story was just too far removed from the characters we know and love from the original X-Wing. Still, he tells a story with fun twists and surprisingly rich character development. As such, I find it very difficult to rate this novel because I think what you get out of it really depends on how immersed you are in the EU. For me, it was about 3.75 stars - a good read, but not up to the level of the original X-Wing novels.
So, I was lucky enough to read an advance copy of Tim Zahn's Scoundrels, and for various reasons it forced me to revise my review of Mercy Kill. Mercy Kill was supposed to be the Star Wars espionage thriller of the year. However, after reading Scoundrels, this pales in comparison. I'd originally given the slow pacing and lack of gripping plot twists as part of the espionage genre. However, Scoundrels shows how to do this sort of thing right. By contrast, with Mercy Kill, I think part of the problem is that it's tough to care about the plot. We don't get any major characters from the films and the idea of flushing a general out is just too tangential to anything important. Second, the characters in Wraith Squadron act like a bunch of immature goofballs. Some goofing off is fine, but joking about having to go to pee or calling somebody "Poop Dog" ranks amongst the worst of Jar-Jar Binks' humor. Scoundrels sets a new standard for genre-defying Star Wars and I only wish Mercy Kill had captured some of that intensity.