Is it a sequel, prequel, or reboot? Yes! X-Men: Days of Futures Past conveniently uses time travel to rewrite the already bungled X-Men history and canon, primarily to undo the crap that director Brett Ratner did to the X-Men franchise under his watch. In order to pull this off, director Bryan Singer pretty much had to bring in all the actors from all the movies, including young and old versions of the key characters.
This installment is probably the best of the entire X-Men series. The dialogue between young and old Professor Xavier (James McAvoy and Patrick Stewart) is priceless. There will be plenty to nitpick about with regards to continuity amongst the various installments with different directors, but once you look past these inconsistencies you will probably grow to appreciate how this movie attempts to reboot all that madness.
I give this movie 4/5 stars. If you liked X-Men: First Class (2011) and hated the X-Men trilogy directed by Brett Ratner, you will probably enjoy this movie. If you gave up on X-Men a long time ago, it might be worth looking into this movie, and X-Men: Apocalypse (2016).