Most of us will probably only want to read the first two parts, which are magnificent.
If you happen to be interested in comparing Milton's epic to the Homer or Virgil's, you should definitely look at John Leonard's MA thesis - he's the current president of the Milton society. It's available online, I found it on Google. It's all about how Milton outdoes the heroism of the Greek and Roman epics by making the Son's actions even more incredible, although many later readers found these passages farcical (eg. Ajax throws a rock, Christ throws mountains.)
When Lucifer flies through Chaos, the super interesting possibility emerges that in Milton's cosmology it is primal Chaos, and not Hell, that is God's true enemy. Such a reading places Satan as a dupe and a distraction, infinitely less powerful than the two rival powers that war above his head.
All the passages about Eden and temptation are weighed down, I think, by their need to investigate and support Milton's (frankly nonsensical) theology. If you do read to the end, though, there's a great scene between Adam and Eve... still, though, the real genius is in the first two parts, because that's where Milton's imagination was allowed to run riot rather than support and attempt to logically synthesize dogma.