It may not have escaped your notice that the news of "Uncanny X-Men"'s impending reboot makes "X-Men Legacy" the longest-running Marvel series to have avoided any renumbering stunts during its lifetime -- although it did get the word "Legacy" appended to its title along the way.
Somehow, this fact seems fitting for a series which has always maintained a strong focus on its cast's history, while reworking them for the future. Following "Age of X," the cast of the title (who, in various roles, have been fixtures for several years now) has apparently crystallized into an actual team. Rogue, Xavier, Frenzy, Magneto, Gambit, and Legion set off to apprehend Legion's missing personalities, working out their various tensions. It's incredibly character-driven. Every beat of the plot is calculated to play to the cast that has been established, and it flows beautifully from one moment to another.
Admittedly, if you aren't familiar with some of the characters' history, there are moments that might make no sense. I could be wrong, but excluding any possible meetings while he was posing as an Avenger, Gambit last fought Bullseye back in 2000. It was a defeat that you can imagine weighing on Gambit, but it's hardly character-defining, and 11 years on, Carey is probably playing to a small portion of the audience in referencing it. As a critic, I think it's gratuitous and unnecessary, but as a reader I'm exactly the kind of person who enjoys it.
Of course, if droplets of obscure continuity don't confuse you, Carey has another attempt in the issue's second half, where he picks up the thread concerning Rachel Summers and the missing Starjammers and finally - Finally! - starts to pull on it. It's only confusing because rather than simply tell you what Rogue learned from Rachel's fading astral projection, he does the sequence backwards, from Rachel's point of view. It's a nice little device, which I'm sure was harder to write than it seems to read, and it turns some potentially flat exposition into a strangely memorable and unusual scene. That alone should be applauded.
The anniversary spectacular is rounded out with a reprint of an old "New Mutants" issue, ostensibly chosen because it shows the first meeting of Xavier and Legion. Between the title's offbeat style and the lack of proper setup or resolution (this is a middle part of a longer story), it's hard to imagine there wasn't a better choice than this lying around.
Despite the odd choice of bonus material, there's very little to complain about, technically speaking. It's possible you don't like Carey's story choices -- his attachment to Frenzy, his belief in the Rogue/Magneto pairing -- but if those don't put you off, it's simply firing on every cylinder. "X-Men Legacy" might soon be the longest-running X-book. Issues like this show that it's worthy of the accolade.