Epic Fail
I have to say that as much as I've loved and enjoyed this series, the ending has soured me on the whole thing. This issue was severely rushed, and was reliant entirely on the entire cast making the absolute stupidest mistakes possible in order for Mystique's final plan to work. X-23 completely forgets she knows how to tell when someone is Mystique in disguise (which she revealed in the same series, no less), the only people who bother fighting Siphon are the ones with active healing factors (while those without sit on the sidelines and watch), and they decide the best way to find a shapeshifter is for everyone to split up.
Probably the worst part was that the ending leaves you feeling like the entire story just plain doesn't matter. Not just because of Secret Wars, but because NOTHING ends up happening in the end. The build-up to what looked to be an awesome confrontation between X and Mystique goes no where, and everything just fizzles out.
For a series that began with such promise it's such an anticlimax that anticlimax just doesn't seem to be an adequate description.