I see divided responses on this. I loved it almost unreservedly. I think that if you read the previous X-Wing books and loved the Wraiths the best, you'll enjoy Mercy Kill much more than if you were more of a Rogue fan.
I don't feel able to write a decent review, (too emotionally attached) so instead a few points, things I loved and things I didn't.
This is Wraiths: The Next Generation. So there's a decided lack of beloved characters in leading roles. Wedge, for example, shows up in literally just a few paragraphs, and Tycho is only mentioned. Face has an important, but brief part to play, and Piggy is the only former Rogue or previous Wraith with a big part. No Wes or Hobbie. I am semi-devastated by this.
That said, the new Wraiths are pretty awesome, even if they don't have the same character development that Piggy (now going by Voort) gets. I'm especially fond of Myri Antilles (daughter of Wedge, of course) and Jesmin Tainer (daughter of Kell). Jesmin is her father's daughter, and Myri shows every sign of having spent too much time with Aunt Mirax as a child.
The tone is, for the most part, exactly as I'd expect from an Allston X-Wing novel. You call in the Wraiths when you want hilarious explosions, and they deliver. They have, however, switched from being a fighter pilot unit to being intelligence, so they're a bit more Leverage-y now. On one hand, it makes the X-Wing tag essentially a lie, on the other, it makes sense for the skillsets.
That said, I wish this had been set decades earlier, so I could've seen the original Wraiths transitioning from pilots to intelligence. There's a few brief flashbacks to that effect, and it only makes me want to see that book more. But the EU seems to have transitioned to almost entirely NJO and later or Clones War era material. SIGH.
Speaking of the EU... If you haven't been keeping up with the last several years of Star Wars books (I haven't) and have intention of doing so (not likely) I'd suggest a visit to Wookiepedia to do a quick read about Jacen Solo, the Yuuzhan Vong, and the Yuuzhan Vong War. You don't need to know all of this, and the book does an admirable job of explaining what's necessary, but I think you'll get more out of the book if you have that background knowledge in advance.
Finally, the villain her is far less compelling than Isard. Don't go in expecting a worthy successor to her, because you won't get one. Given that it's been thirteen years since the prior X-Wing book and there's likely no guarantee of another ever seeing print, I wonder if Allston was hesitant to put that much effort into the opposition.
To sum it up... If you read and loved the X-Wing books, especially the Wraith ones, read this now. If you haven't read the other X-Wing books, why not? They're among the best Star Wars novels, and just about the only thing that could break my decade-long avoidance of the EU was a new installment.