I liked this story in the sense that I thought the language was quite well done and I did like the idea of the story, a parasite being placed in the midst of a household in this manner.
However, what I suffered from here (as I do with most older classic horror stories) is the lack of immediacy in the writing. I have to work extra hard to place myself in the position of any of the characters which is a shame because in this case I would have found some scenes quite chilling.
This is not the fault of the novella as it's a product of its time. I have similar problems with H G Wells and Henry James, I have to work hard to get the atmosphere intended in the telling.
I can see why people would have issues with the explanation part at the end, but for me it seemed like it wasn't the point of the story. To me, the point was the chilling bedroom scenes, the insidious nature of Carmilla's friendship with Laura. So it doesn't surprise me that this could have been a cautionary tale about lesbianism or something similar.
What is that strange girl doing with your daughter behind closed doors?
Anyway - I liked it, it was a quick read, but I was a bit disappointed that I didn't get the impact I would have liked from it.