I have no particular problem with Marsden. I'd never heard of him before X-Men, but he seemed just fine to me, even as someone familiar with the comics. I had more of a problem with Famke as Jean, though I liked her much better in X2. I don't dislike Famke, mind, and I liked the alterations they made in the character, taking Jean from X-Men "den mother" to professional 30-something woman. (I was always a fan of Jean, back to my childhood -- and I'm Famke's age.) But Janssen wouldn't have been my first casting choice; not sure I actually had someone in mind, but Famke startled me in a way that Marsden didn't. I thought he nailed Cyclops. I felt as if Janssen needed more time to find Jean.
Ironically, I think a better Jean might have been the actress who played the doctor on the old Star Trek series with Patrick Stewart, but that would have been a *really* strange cast, given the history of those two actors (not to mention she's now much too old). But in looks, I think her closer to Jean.