Now this was a good Marvel movie.
In terms of tying it into the other X-Men movies and the X-Men universe in general there were continuity errors, specifically with Beast and Havok, but you are able to set them aside to enjoy the movie. Magneto was a total and complete badass and I loved every minute of it. Michael Fassbender was the perfect choice. Professor X was also very well done and well cast.
The one thing I will admit I did not like was Mystique. I don't know if it was Jennifer Lawrence or what it was, but I could not stand the character. They tied her into Charles Xavier in kind of a weird way showing she was a runaway who impersonated his mother and got caught. She was taken in by him and they wound up going to college together until the Mutant uprising where she disappoints Charles and then joins Magneto in the end. I know Mystique was always kind of whorish and seductive, but it did not work in this. Between her trying to seduce Charles and then moving onto Hank while also trying to accept her natural form... I don't know. It was weird.
As for continuity, if you remember X2 you will remember Hank McCoy (aka, Beast) appearing on the TV in normal human form. First Class takes place in the 1960's where Hank is a young, brilliant scientist working with the CIA. He comes in contact with the other Mutants and after meeting Mystique comes up with a "cure" to the physical deformities of his and her mutations. He tests it on himself where he becomes Beast. A big, giant blue werewolf type guy. Now... If he was transformed into full mutation in the 60's, how would he be a normal man in X2? Unless he came up with something to actually cure it in the meantime but I think that is a stretch given the way the Mutant protests were portrayed in the previous X-Men movies.
Another minor one was Havoc. Anyone who knows X-Men knows that Havok is Alex Summers. Summers is also the last name of Scott (aka, Cyclops). Cyclops was in the previous movies played by James Marsden and is a young man in those films. Alex is portrayed as a troubled young man who was taken out of prison to become a CIA Mutant by Magneto and Professor X. How could they realistically be brothers if Havok was the same age, if not younger, than Cyclops was in X-Men back in the 1960's? That makes no sense unless they just don't care to follow the family lines of the comics.
Overall we learn how Beast is born, learn more about Magneto's past and why he was so against peaceful integration of Mutants and humans, how Professor X is put in the wheelchair, how Cerebro comes into play, etc..
I did enjoy it. I honestly consider it to be the best film out of all 5 X-Men movies made so far. I didn't let the continuity bother me.
Oh, and there is a Wolverine cameo. Just fyi.