This is a fascinating debate that has caused debate since its publication about how precisely readers are supposed to assess it and what meanings they are to take from it. On the one hand, it is possible to argue that Book I, because it begins just after Satan's failed revolt and when he and his minions have been cast down into hell, presents Satan as a character whose arrogance and self-belief is so exalted that he immediately thinks of revenge and is unable to accept that victory is something that will continually elude him. Note for example how he looks at his angels and thinks that they are so powerful they will eventually defeat God and his forces of good angels:
For who can yet believe, though after loss,
That all these puissant legions whose exile
Hath emptied Heav'n shall fail to re-ascend,
Self-raised, and repossess their native seat?
Arrogance and distorted vision characterise him, thus showing the portrayal of Satan as an evil character who demonstrates the consequences of sinning against god through trying to be more than he actually was. Note how Satan views his army as "puissant" or powerful, even though they have just been defeated. He also exaggerates with the phrase "Hath emptied Heav'n," whereas in fact we find out later his troops only number a third of the angels in heaven. Conventional assessments of Book I present Satan as an evil character who has received exactly what he deserved through his sin.
However, at the same time, other critics have argued that Satan strikes the reader as an intensely charismatic and appealing character, in spite of all of his faults. Some indeed argue that Satan is the much more appealing character compared to God, who is frankly a bit boring in this epic text. A quote that many use to indicate Satan's appeal is how he determines to accept his new position and use it to possess the power he was unable to seize up in heaven:
Better to reign in Hell, than serve in Heav'n.
So power-hungry is he that he is willing to settle for life in hell, where he can rule, than return to heaven, where he would have to serve. Such an assessment of Book I therefore focuses on the way in which Milton began his epic not with God or Jesus, but Satan, and how he presented him as an appealing character whose powers of seduction win over the reader in spite of him or herself.