Shelley believed this to be his masterwork, and it certainly is his longest and most ambitious. Even so, I mark it my least rewarding of all I have read of him. The reasons are numerous, but in general: a presumptuous argument, characters drawn too ideally to make that argument, and so much extraneous imagery and dithyrambic speech that the poetry is compromised.

To be certain, the resetting of the Aeschylus story of Prometheus in Shelley's Christian setting is challenging enough; that Shelley hoped to also make it a story of faith vs. reason merely raised his stakes. This makes the parallels between the characters often convoluted; Shelley himself introduces several (most notably the Demogorgon--not to be confused with any similar-sounding name you may have ever heard before in mythology, role play, or Stranger Things) in order to better define his allegory. Yes, the Promethean/Miltonian kernel of rebellion against the divine arrogance is here, but where Prometheus was himself first a collaborator in raising Zeus/Jove to power and still himself bears pride and ego--and where Milton's Satan holds some remarkable arguments based on reason tainted by pride and deception--there is neither subtlety nor roundness to Shelley's characters: Our new Prometheus is wholly virtuous and sacrificial (indeed, he might have learned a bit from Mary Shelley on this one). The Demogorgon is the glory of reason and free will. Jove is all tyrant and selfish power, etc. 

Once freed, the artist of reason may now (somehow) be free to create and fully realize ambition without the distraction of pain or suffering. Briefly, I find this claim truly problematic, but Shelley presumes it without elaboration. 

Where these ideas and difficulties might have been wrestled with in earnest, they are offered only in the closing act of the poetic drama, the first three spending more time in retreading mythological ground and offering lengthy exclamatory images of cloud and mountain and the like, none of it near the poetic achievement of his shorter works.

I am grateful to have read it, to have the contextual space to contrast it to Milton, Blake, Aeschylus, Augustine, Mary Shelley, Wilde, and others who work similar questions. But neither in its poetry nor its theme does Shelley match these thinkers.