I'm already seeing some people make the excuse for this movie that it was super-frontloaded on its first weekend. While yes, we can only expect to see at the very least a 45% drop for a movie that grosses 100 million in its first weekend, previous movies (and very recent) movies that have done the same did not drop this low.
Take Revenge of the Sith for example. That movie grossed around the ballpark in its first weekend (108 mill) and dropped only at 49% its next weekend. That movie had for the most part a very positive word of mouth. Then there's this movie. Yea I heard people say it was good, but the amount of people just bashing this movie is insane.
Obviously the insane deviations from the source material has hurt the movie. People have to remember, the people who give a movie repeat business, especially for franchises like this, Star Wars, and Spider-Man are fanboys/fangirls. You need to please them to a certain extent to make sure you please your repeat viewers. A lot of the fans have felt that the movie completely botched the characters and story of the Dark Phoenix (one of the most beloved comic tales). Even people who enjoyed it, said that they didn't like the alterations. However, they don't plan on watching it again. So go figure.
And its just not this movie's bold deviation from the source material that is hurting it. Its the A.D.D. concept of the whole movie. A lot of people say the movie is too short, and its really rushed.
I don't think the movie is horrible by any means, and who knows I'd probably just own it on DVD to be a completist as its not that bad, but it is SORELY dissappointing. Hell I haven't even seen it a second time, and this is coming from a guy who saw X1 five times in theatres, and X2 six times. The previous Xmen flicks got repeat business because the movies were finely crafted (though X1's story was a bit weak, you can tell the makers tried really hard to make the movie as good as possible)