Well, she made an effort. And she raised interesting issues, though she never really resolved them. But I think Diane Duane works better with a smaller cast. Her original novels are fine; they have much smaller casts. Her Spiderman trilogy is fine; it has only three main characters. Even her Star Trek novels are fine; she tends not to use more than five major characters per book. In "Empire's End," she's dealing with Professor X, Beast, Gambit, Wolverine, Storm, Bishop, Iceman, Cyclops, Jean, Lillandra, a Shi'ar lord, etc. There are simply too many characters, and though each gets a few scenes or "moments," they all seem shallow.
The plot is as follows: the Shi'ar discover that a mysterious creature, which destabilizes galaxies by eating their cores, is heading their way. They enlist the X-Men to stop the creature. This totally unequal confrontation is made "plausible" by a strange device that enhances the X-Men's powers. Gosh, how convenient. Meanwhile, Lillandra's council is pressuring her to marry a Shi'ar noble and produce an heir, instead of continuing her relationship with Charles. Ooh, look, a romantic subplot.
There are some interesting discussions about the effects of absolute power, and the enhancement of the X-Men's powers at the expense of the more human aspects of their personalities, but when they finally save the day (you never seriously thought they wouldn't, did you?) those issues vanish like mist, never fully confronted. The power-enhancing device also conveniently disappears; I sense an author avoiding the implications of a bad concept.
Also, I'd really like to know exactly what the huge, ghostly X-Men-shaped constructs are. They are first described as psychic projections, or something along those lines, but later seem to become real. They seem to be a device for avoiding the idiocy of pitting the X-Men against a creature larger than Manhattan. I say, if you're using the X-Men, give them an adversary they can confront as is. But if you lack the foresight to do that, the least you can do is remain honest to your original premise, and keep the relative sizes of the space creature and the X-Men as is. (Actually, the ghost figures aren't needed for the final victory, which makes me wonder even more why they were there in the first place. I sense more bad concepts.)
The story is fast and fun, and Duane's style is quite readable, but "Empire's End" feels like a rush job by an author not quite familiar with her world and characters, who tried to make up for the gap by shoving them into Star Trek.