Bryan Singer, welcome back to the X-Men fold. I enjoyed watching Days of Future Past. The movie, with all its many different plot points, manages to flow along smoothly and I don't think any part of the movie felt dragging. It's about as good as it gets with the usual summer blockbuster fares. If you enjoyed First Class and the Singer's other two X-Men flicks (2000's X-Men and 2003's X2) back then, you'll definitely enjoy Days of Future Past.
Some spoilers below. Just a heads-up.