Personally, I thought it was awful. Hands down the worst of the three. None of the three or four "mini-plots" connected with each other with anything more graceful than a train wreck. We have a teenage soap opera (Bobby, Kitty, Rogue) which goes absolutely nowhere. We have Magneto recruiting mutants from the local biker gang to kidnap a little kid. We have Jean coming back from the dead to kill off characters because she has a latent obsession for mindless destruction. And we have the Angel subplot, which was included only to show off a wide-angle wing-unfurling. None of these plots are interesting alone; nor are they interesting when they're smashed together with special effects and pointless cameos. And they have to be smashed together because none of them depend on any of the other to work. Magneto didn't really need Jean to kidnap a kid, Jean doesn't need anyone to rip stuff apart, and the Bobby and Angel subplots are ridiculously pointless.
And the two main subplots were awful on their own, too. What was Magneto (who apparently is now a lunatic rather than a misguided genius) doing since X2? Sitting around drinking coffee until he heard about this cure, got really pissed, and decided to take it down so he can go back to drinking coffee? Since when is the evil overlord's master-plan a simple reaction to something? What would have happened if Magneto stole and killed the cure-kid? The world would go back to the status-quo established between X2 and X3. That doesn't exactly make for epic suspense.
And the "Phoenix" was embarassingly bad. Jean turns into a super-powerful, child-like zombie character similar to the ridiculous "Buu" in Dragonball Z. We're a long way away from the maturer storylines of socio-political conflict that makes X-Men so great.
Now that I think about it, DBZ is a good comparison. X-Men 3 basically reverts to DBZ. Characters are killed off, but don't worry, they'll "come back." Powers are taken away, but don't worry, they'll "come back." Ugh!