Assemble these ingredients: 
 
Great cast - especially January Jones who really sold Emma Frost without being slutty.  

Grounding in reality - Magneto is a Nazi hunter, Beast is an electronic engineer with the CIA, Charles Xavier is a professor who regales those in bars with his theories on mutation and later joins the CIA, and Shaw and Frost are basically Bond villains in a pirated sub! Okay, not so grounded, but rock solid for a mutant movie.
 
>>Dare I say it?<< NEW MUTANTS, not the REAL first class - Yes, I thought it was a mistake going in, but now I think sitting still for "what's wrong with my eyes?" and "I seem to be reading minds!" from Cyclops and Jean Grey, and not-so-subtle hints that they will hook-up were a welcome deletion from the film. Which leads me to... 
 
Shifting alliances - No spoilers, but let's say several characters good and bad, switch viewpoints and teams, but always for a reason. Only in one case is this a mistake in that it is something the audience can't buy (you'll know it when you see it).
 
Mix well, and you have the best X-Men movie ever.