This is an all-time world classic and well worth the extra effort. The language may be somewhat unfamiliar to most but will be readily understood with a bit of patience.
Well the first thing to remember is that Milton (the author) was a Protestant. Therefore, he necessarily makes doctrinal errors when discussing theology. Now, I do understand that this is meant to be a novel so there is some poetic license. However, Milton, speaking as a Protestant, does contradict some fundamental points of doctrine and that could be very confusing for someone who is not steeped in Catholic theology.
Paradise Lost is actually an epic poem which was kind of cool at first but gets kind of annoying after a while. The language is difficult to understand and the poetic rhythm gets a bit repetitive. And the book just seems to drag on. By the end I was just sick of it.
Honestly, if I want to read about the fall of man, I'll pick up the Bible.
"Yet live in hatred, enmity, and strife Among themselves, and levy cruel wars, Wasting the earth, each other to destroy - As if (which might induce us to accord) Man had not hellish foes enough besides, That day and night for his destruction wait"
"in mercy and justice both, Through Heaven and Earth, so shall my glory excel, But mercy, first and last, shall brightest shine"
"How few sometimes may know when thousands err"