Without going into much explanation or detail about the time-travel element of "X-Men: Days of Future Past", I will say that if you have seen other time travel movies (more notably "Back to the Future" and the first two "Terminator" films), you will know not to think too hard about the impossibilities and intricacies involved. Your head will explode.
What "Days of Future Past" does well, though, is make relatively easy to understand the steps involved in fixing a giant problem with the downfall of the human race by the giant mutant-hunting robots called Sentinels. The process, in the realm of the X-Men universe, seems rather doable considering one of the last remaining mutant's powers involves not necessarily "time travel" as we have come to know, but "transferring consciousness from one body into the body of an earlier self". In the midst of annihilation, Wolverine is chosen to be the traveler because he is the only one powerful enough to handle the transference, and is sent back to his un-aged 1972 self with orders from the old Charles Xavier and Magneto to rally up their younger selves to prevent the assassination of the creator of the Sentinels (played by Peter Dinklage) by the hands of shape-shifting Mystique, which in turn is the event that spawns the U.S. government's orders to create the Sentinels in retaliation.
Phew. Just writing that was more exhausting than thinking about it. Seriously, while watching the film, returning director Bryan Singer and writer Simon Kinberg do a rather flawless job of setting up the circumstances while having fun with the Wolverine character as he tries to convince young Charles Xavier that he needs to become the man he is destined to become in order to help save both humanity and the mutant race. The film makers don't spend too much time on the errs involved with certain aspects of time travel, because this isn't necessarily "time travel" we are dealing with.
The story itself is interesting, leading for some unique character moments between young Charles and Erik (who was apprehended after the events of "X-Men: First Class" for his involvement in the JFK assassination, who as he put it, "was one of us"), and for Logan/Wolverine to intervene on their older selves behalves in order to gain a sense of trust and dire need of duty to prevent a future apocalypse. Again, as with "First Class", James McAvoy and Michael Fassbender flawlessly recreate the buddy/enemy relationship that Patrick Stewart and Ian McKellan started so well in their portrayal of the characters.
One of my favorite moments in the film involves the new character Quicksilver, who can move so fast that a speeding bullet looks as though it is inching at a snail's pace. The film slows down all the action amidst gunfire, while he (in what to us now looks like normal speed) moves around the room and manipulates the events that are about to transpire in real time. What results is one of the coolest slow-motion visual effects I've seen, and even outdoes Bryan Singer's own "X2" opening sequence involving Nightcrawler's teleportation.
What makes the film unique is not just in how it handles it's "time travel" scenario (in which future Wolverine's consciousness is being projected into his earlier self, but all havoc could be unleashed if the body of his future self is harmed...which is constantly in threat with the incoming Sentinels), but in how director Bryan Singer is able to wrangle all of the "X-Men" movies together and somehow reboot his own series by restarting some scenarios, and even re-introducing certain characters who may or may not have been killed in previous films.
What we get in "X-Men: Days of Future Past" is not just another Bryan Singer X-Men film, but a genuine sequel to both his "X2" and Matthew Vaughn's attempt at reboot with "X-Men: First Class". So, for what could be a first in film history, we are essentially rebooting a reboot in order to get back on track with a series that was, at one point, in a position to be rebooted, and at the same time using all the materials of said films at its disposal. Try to think about that one for a moment without crapping yourself a little.
Hugh Jackman is a freak of nature, looking in better shape from movie to movie, and the acting, especially on Jackman, Stewart, Fassbender, and McAvoy's part, is superb. Jennifer Lawrence's Mystique is the central antagonist and the reason all hell has broken loose, but her character is not the only true villain of the film. There a villains left and right, new and old, and some with a wink to previous story lines (Major Stryker, especially).
The one idea that I really enjoyed that Bryan Singer included in the film is something similar to what James Cameron introduced in "Terminator 2" - essentially the "no fate" scenario. Basically, what "Days of Future Past" is trying to claim is that no matter what you do to change the course of the future, in some way, the ripples of time with find their course as do ripples in water, and eventually time will find a way to make certain events occur. Certainties are certain to happen.
I also liked that Singer's crew returned with him, including cinematographer Newton Thomas Sigel, who has a way of making every film of Singer's look as though it exists in "The Usual Suspects" universe. Composter/editor John Ottman also brings back the theme from "X2" and keeps an eye on the action, always letting us see just what is happening (which is great considering two characters can control objects around them whether through magnetic control or speed control).
Many of the stars from the original X-Men films make a return (mostly in cameo), as well as a few surprise turn-ups. And stay for the end of the credits, for a scene that sets up the origin of an eventual villain in the next X-Men film, tentatively titled "X-Men: Apocalypse").
Easy-to-understand "time travel" scenario. Dark, foreboding, and ominous tone in the future scenes. Unique character moments involving "X2" cast and "First Class" cast. Cool 70s setting. Giant, murdering robots. Jacked-up-looking Hugh Jackman. Great subtle acting moments from both Charles Xavier actors, James McAvoy and Patrick Stewart. Winks and nods to future/past X-Men events, that have already come to pass and also, due to changes in the space/time continuum, have yet to occur. Good, melodic musical score. Easy to understand action. Entertaining mutant moments. Slow middle section that still finds time for good acting and useful dialogue. And a nod to what could be considered the first ever TIVO, created by Hank McCoy ( aka "Beast").
Overall, probably the best "X-Men" movie, but as of now nearly my favorite (I still have a soft spot for "First Class").