I can't say I'm a huge fan of the series, but I saw all 3 other films, apart from Origins: Wolverine and have played the games for MegaDrive way back when and saw a few episodes of the cartoon series as well, but it seems to me that I know more about each character's individual story than the director who did this...
For starters, to "Americanize" characters who have trademark accents or nationalities seems to me to be utterly supremacist or racist, to say the least, not to mention it completely ruins the feel for most fans of the whole franchise that know exactly the way characters are and what they are known for.
Then come the inevitable love stories that grow out of nowhere, only to be left unresolved. I may spoil a bit here, but had never, in my life, dreamed that Mistique and Beast had once had somewhat of an affair and that Magneto was also interested in her sexually. Then came, the mythical Professor X vs. Magneto relationship, that at times felt like a confused homosexual affair. The soundtrack was fun and the introduction of all the characters in their beginnings felt refreshing and to know that they themselves came up with their iconic names and not some organization branding them was fun too.
The acting was nice throughout and the make up effects were quite noteworthy too, especially on Emma Frost's "Diamond form" behalf.
What some people believe to be the main trump card of this movie though, the special effects, completely had me laughing as they seemed so fake that it is unbelievable how bad they are. Like when the trees get smashed by some nukes or something falling on top of them and then instead of breaking or smashing like any normal tree, they actually bend sort of like rubber, and that was amazing...
I believe you should see it only if you are a die hard fan of the series and really, really want to see every movie and are looking for the prequel. People looking for fun will probably find something funnier elsewhere.