I think it was pretty good for a comics movie as well, and definitely better than the source material. It was also the best version of Dani's powers, which if they'd done that in the comics they could have spared her 40 years of shifting powers nonsense and eventual de/re-powering. I liked what they did with Cecilia's powers too even if they did make her a villain.
As to why it was panned, I think the years of hearing about it was probably the main factor. Even though it was much better than I expected it was still unnecessary. Still I'd put it second best after First Class in that batch of X-movies if we don't count Deadpool 1/2.