how well does it fulfill its purpose?

I think that like his famous short stories, Poe is successful in walking the line of clarity between sanity and insanity. In this case, the protagonist hovers between his desire to forget and desire to remember. The narrator passes through the manic cycles that we have come to expect from Poe. He becomes "weak and Weary", regretful, sad....all the way to madness. So, yes the Raven is successful; it is classic Poe.