I think it's a very fun show! The setting of the show is a somewhat interesting thought exercise. But don't get me wrong, the stuff about portals and alternate dimensions was being hinted at since the very beginning. If you care about story writing, the arcs of John Smith and the Japanese lads are very interesting to follow as well.
A final thought about the final season:
spoiler
It felt just a bit too rushed toward the end. For instance, it's almost like they felt like the Black Communist Rebellion couldn't ultimately win, otherwise the cattle would be asking questions.