First of all, the other review here is of course for the hardcover edition, so bear that in mind folks. Onwards....
I've been a big fan of Morrison ever since his ground-breaking arc on Doom Patrol (in my opinion the greatest superhero comcis story arc ever, including 'Watchmen') and thoroughly enjoyed the first two volumes collecting his X-Men run.
Unfortunately, this third volume doesn't live up to the expectations created by the earlier arcs - Morrison's handling of Magneto is nothing special, we get the whole Phoenix thing again, nowhere near enough Emma Frost and a 'Dark Beast'/future scenario which is...quite frankly, dull. Sometimes studied whackiness isn't enough, and structurally, these arcs are weak compared to Grant's usua; standard.
The art is wanting too, featuring the unwelcome return of Image-kid Marc Silvestri, a vintage but massively over-rated former X-Men artist who is a kind of bargain basement Jim Lee, but a little less scratchy (of course if you like Lee, MCFarlane and that crowd, you'll love this - which probably means you're not a child of the silver/bronze ages of comics).
Instead, do yourself a favour and buy all six volumes of Doom Patrol - yes, it may have influenced X-Men a little (the team first appeared about 3 months prior to Marvel's Mutants), but I think Doom Patrol owes much to Fantastic Four. In Grant's DP, you get masterful plotting, wild ideas, structural integirty and great art, plus some of the most stunning ret-conning in superhero history.