Okay, since you asked in teh Poison Ivy thread, here's my opinion on it.
I'm breaking it up in sections though and I'm giving my opinion as if it was Marvel Studios starting fresh with the franchise and not god awful Fox continuing with theirs.
So save for Gina Torres, you kept the choices from Fox, which other than Patrick Stewart, are easily replaceable and should be in a new movie. I also think Stewart should be replaced, but can't think of anyone else. At least Fox got him spot on. As for Gina Torres, I like it, she's my choice for Storm in a new X-Men movie as well.
Kelsey Grammar has the same prob as Patrick Stewart as I said above; Perfect casting, wrong studio to make the movie with. Holloway is a terrible choice for Gambit and should really stopped being mentioned because of his supposed "acting" on the show Lost. I totally dig Dushku as Rogue, again one of my castings for a new X-Men movie. I liked Cumming as Nightcrawler, but again, if I were in charge of casting a new movie, I'd be picking all new actors. I don't think Charisma is the right fit for Emma. And Gerard Butler is not a good choice for Havok at all; He's still too much of a flavor of the week actor.
Chris O'Donnel as Iceman, I don't know. I've never been too impressed with him and he's not quite how I envision Iceman. Dominic Purcell is an interesting choice, I find that Colossus is a really hard character to cast, so I can sorta see it. Ellen Page didn't impress me as Kitty, she can act, but she isn't how I envision Kitty either. Kea Wong, I don't really know much of her work, so can't really comment on that, but again I'd want fresh actors, and I'd go for Brenda Song in this role. Jude Law is an interesting choice for the role of Angel, which I can see working fairly well.
Kate Beckinsale is a terrible actress, hence no to her as Psylocke. I love Amy Adams, but she's too sweet and innocent to portray Polaris. I know she's an actress and its her job to act, but unless she changes her voice dramatically and tones down her movement, I just don't see her working as Polaris. I don't quite think NPH works for Northstar either, in terms of looks mainly, acting he's ok, but I'd have to see more of his work to really have an opinion on him. Morris Chestnut is good casting, I'm just not a fan of the character Bishop. McKellen is in the same vein as Stewart and Grammer; Can't think of another actor for Magneto though.
Phillipe is more of a Havok to me in terms of casting, though I guess I could see him as Quicksilver as well. Amy Acker as Scarlet Witch; I'm really having mixed feelings about this, I mean I love Acker, and I believe she can do the role great, but something in the back of my mind isn't sure. Sean William Scott is an interesting, original choice for Pyro, which I can actually see working well. Recasts for Mystique, Sabretooth and Toad though.
I'm guessing you're going the X-Men Evo route with Avalanche, as would I with the character. I can sorta see Somerhalder as the role, but I think there would be other actors better suited for the role. Juggernaut should is another in the Stewart/Grammar camp for me; Jones to me was great casting, but the script and costume needed work. Kevin Durand, I've never seen his work, but I see he's playing Blob in Wolverine, so recast for me there as well. I loved Kelly Hu as Deathstrike, but for a new X-Men movie, I'd go with Maggie Q. Julian McMahon, though I love him on Nip/Tuck, he just doesn't work in comic movies for me. I do like Fishbourne as Apocalypse. I hate the character of Deadpool and I find Ryan Reynolds annoying. With Lynn COllins, it's the same as Kevin Durand for me.
For Forge, I know Benjamin Bratt was a popular choice forever ago, nowadays I don't know really. I wasn't too interested in his character that much.
I hate Cable, and feel he shouldn't be introduced anywhere outside the comics and that his existence in the comics should be erased.
And you have Kea Wong as Jubilee abov.