This was a most amusing book that manages to make you laugh and think at the same time. I felt very sorry for poor Wormwood, though I didn't need Screwtape to tell me he was seriously screwing up. I'm tempted to say that the last letter in the book was the best of the lot.
Though I don't believe in demons or devils or whatever it is that they're supposed to be, I find myself questioned my motivations more now than I did before. As much as this is a work of fiction there is a certain amount of truth underneath, as in all CS Lewis's work.