Well, the poetry is quite good. But there were a few parts that fell in the action department. Paradise Lost is much, much more interesting than Paradise Regained. I feel this is possibly because Milton made Satan to be such an anti-villain that you couldn't at least blame him for rising up over an overbearing monarch.
There may be some values dissonance here as well, some sources say this is a very Calvinist book, which is not as popular a view nowaparts.