I LOVE the Xmen. Its tone and low key approach to their powers was brilliant and the cast was perfect. My only criticism would be that the final battle seemed a bit stilted and anti-climactic.
X2 was a slightly better film but I would have liked it more if they'd stuck with low key powers instead of ramping up the explosions quite so much.
I agree with Christopher that the biggest criticism is that it totally wasted the Phoenix plot, which should have been the A-plot without a doubt. I thought it was unnecessary to have such a large crowd of mutant cannon fodder at the end too. Apart from that, I really liked the X-group dynamic at the end and I thought that the Beast's plot was a very good, self-contained arc.
I'm totally up for an X4.