Don't read this if you haven't seen X-Men III yet (unless you don't care).
Okay, it really pains me to say it, but I hated this movie. I kind of knew going in that it might be a disaster, and I sort of knew many of the key plot points, but I still had hope that I'd enjoy it. Especially since I was SO looking forward to it after X2.
I used to (admittedly) read the X-Men comic books, so I'm a bit of a fangirl. My reactions, therefore, stem more from that angle than from that of a casual fan. Because as an action movie unto itself, it was actually good, and I had no problem with it. It was entertaining, which usually is all that matters.
There was some cheesy dialogue though, that was pretty cringe-worthy (like, "I'm the Juggernaut, BITCH!").
My big problem was with the deaths. I'm not even a big Cyclops fan, but to have him just summarily killed off like nothing, when he was such an integral part of the Phoenix Saga is horrible. I totally understand the marketing reasons behind making Wolverine the hero, but it just messed with canon a little TOO much.

The scene between Wolverine and Jean in the infirmary was pretty hot though.
I really didn't like Xavier's death either (why did they need to kill off SO many people? And cornerstone characters, at that.). That was actually the turning point in the movie for me. I just hated it.
They did do a good job with the Xavier/Magneto relationship, though. And it was cool to see the more sinister side of Professor X.
My last problem was not seeing more of the Phoenix. It seemed like Jean just stood around for the most part, and that they didn't really know what to do with her until the end. There was a lot of potential with that storyline, and I don't think they tapped all of it.
Rogue taking the cure didn't bother me in the least, because I think the character hasn't really worked since the beginning, and Anna Paquin just bugs.
Maybe I need to watch it again, I dunno. Kenny actually disliked it so much that he wrote to Marvel to complain, LOL.
We're such nerds.

Agree. I actually hated that they already showed Jean in the previews ... lol ... but moreso they totally messed up the Dark Phoenix storyline! And my James Marsden ... :(