It wasn't that bad. It wasn't great, it was average at best. However, since Marvel has progressively raised the bar it makes average movies come off as bigger let downs. Plus, this X-Men movie made the mistake of releasing very shortly after Civil War did. Big mistake because that movie was great and following it raised the bar up even higher.
The characters were fine, but the fight scenes weren't that compelling and the story was just too generic. Plus they changed way too much from the comics. Making Mystique the lead just b/c it's Jennifer Lawrence was a poor choice. Same with changing the origin of how Weapon X gets loose.
IMO, ranking the movies: Days of Future Past, First Class, X-Men 2, X-Men, all the Wolverine movies, then Last Stand which was putrid. Oh, and if we want to count Deadpool then obviously move that to the top of the list. Good point too about Deadpool only getting greenlit because someone was wise enough to leak the test footage and it went viral.