The book is an enjoyable and easily manageable read. Each letter is a concise and snarky summary of Lewis's religious convictions. You might be initially drawn in by the self-satisfaction with which he complains about "people these days". However, you'll also notice the misogyny and apparent distaste for social justice.
The last chapter, which is not Screwtape's toast, is by far the most compelling. It beautifully portrays eternal salvation in a way that could inspire a believer and sadden an atheist.
I give it three stars because I found it interesting enough to finish, not because I agree with Lewis's worldview.