I will try my absolute best to ensure I leave out any spoilers and simply focus on what made the movie good and what made it bad. That said, here is my review:
X-Men: First Class, the fourth or fifth installment in the series depending on whether or not you count Wolverine's movie, manages to find a niche unto itself which the other movies had not covered. In the previous movies the X-Men were already well trained, in control of their powers and keen to fight for their side. Whenever there was a battle, the audiences focus was on the ability for one characters power to beat another one's power (think this scene).
X-Men: First Class, being a prequel, takes the concept of powers in a different direction. We see internal struggle, with mutants having to overcome their own limitations, fears and humanity to rise above the rest. Those aspects made this film great, and allowed the audience to connect more with the mutants on an emotional level. This connection kept you distracted from the at times massive suspension of disbelief necessary to reconcile some of the situations.
In terms of the plot, the story builds slowly, adding in cameos and explanations to the origins of some of the characters from the first three movies. The other neat thing about the plot is that it does a great job of integrating actual history with the "what if?" of mutants existing. Speaking of mutants, so as to not spoil anything let's suffice to say that the powers in this movie are pretty cool and definitely subtly enforce the characters flaws/character traits without barging into centre stage.
I give this movie a hearty 7.5/10.