Well, I just finished the entire campaign of Peace Walker (on solo!). It was definitely a great experience. However, in my opinion, this game is not the "MGS 5" that Kojima claimed. In retrospect, the setting for Peace Walker is most definitely a side-story, just like Portable Ops. In fact, I'd even go as far as to say, that the Peace Walker chapter isn't needed at all for the MGS time line.
Some questions were resolved regarding the Boss and her final mission, however, it's not like we needed Peace Walker to tell us those things. Those specific questions were already answered for us in MGS 3 and MGS 4, but they just needed some digging around.
Don't get me wrong, Peace Walker is still a great MGS game, it's just that I wasn't expecting for it to be another side-story. But, I will say, that after finishing the game, I have a lot more respect for Big Boss' character. He is a true revolutionist, hero, and anti-hero all in one, battling against the times, against a world gone mad.