The voiceovers at the beginning and end of X-Men: Days of Future Past are some of the best in film.
Part of the importance of having good voiceover narration is that more often than not, movies fail at this. I'm thinking about the types of voiceovers that require the main character to simply announce what it is that they're doing while they're doing it. The cinematic release of Blade Runner was guilty of this, where the two lead characters drive away, and the voiceover informs the audience that, yes, the two lead characters are indeed driving away. Riveting stuff.
Less offensive but still pretty bad is when voiceovers inform the audience about the world that they're just entered at the start of the movie. Sometimes this gets sidestepped a little bit and has instead the character talking to another character to explain to the audience by proxy. This is Narnia. There's an ice queen and she's bad, but we have a lion and he's like Jesus, but you're not supposed to mention how obvious that is.
So the DOFP voiceovers at the beginning and end of the movie avoid these pratfalls fairly well. The initial voiceover is Future Xavier saying (as in the trailer) "The future...a desolate place" and then some other stuff and then "but can the future be changed?" The ending voiceover is Past Xavier saying "The past...an uncertain" something something.
First, this is kinda cool in that you have Future Charles as narrating, guiding the audience, in the future and Past Charles doing the same in the past. That is...cute, but it's not the reason that I like this voiceover so much. The X-Men movies have always been very versatile in what themes that they choose to emphasize. X2 for example really played into a lot of the persecution that mutants receive in the X-Men universe and drew a lot of ties to real-world repressed minorities. First Class emphasized more of an "us vs. them" narrative and the different ways to approach that. (Having Nightcrawler wish that he could look normal in X2 is one thing. Having Beast change his appearance out of shame for his big feet is another.) First Class also reminded us that X-Men is about teaching as much as it is fighting. Watching characters grow stronger and control their powers better exemplifies them growing as characters. Very few movies have a way to visually represent that to the audience.
With a few different themes floating around in the X-Universe, it's important to make it clear what the audience should take away from this specific movie. When I first saw DOFP, I was confused why MLK or Malcom X have not made an appearance or been mentioned in any of these movies set in the past, as the issue of prejudice against another group of people is in the forefront of my mind when I see an X-Men movie. But, I misunderstood the intent of Bryan Singer. The voiceovers, especially the last one that we hear, are about hope for change. It is on full-display in the way the characters are dealt with and the plot progresses, and the voiceovers simply clarify this even further.