Because Christabel is unfinished, and especially because there were supposed to be at least three more parts, reading it feels like getting sucked into a story only to find out that all the pages after the prologue and the first chapter are missing. We never know what happens to Christabel and who or what Geraldine really is, but leaving the poem unrated just because the story ends abruptly would be a bit unfair. I mean, something can still be said about the language, the atmosphere etc.

I absolutely adore The Rime of the Ancient Mariner (1798), but although Christabel is a more conventional horror/gothic poem with a linear narrative, it doesn't mean it's bad. Just that it's entertaining but not particularly remarkable. The second part is much less interesting as it's a sort of an interlude that leads to the following events (whatever they may be), whereas the strongly gothic first part is remarkably similar to Carmilla (1872) with its heaving bosoms, supernatural elements, and two ladies having a very close encounter.

Although I picked Christabel up because I've heard about Geraldine in the context of vampires, I hesitate to call it a full-blown vampire story. That's just because the poem is unfinished, so we can't really be 100 percent sure about what's going on despite the relatively strong hints. Who knows what surprises Coleridge had in store (and I hear he had too much of them, because he couldn't decide how to end the story).