I suppose the primary interest of this novella is the fact that Bram Stoker almost certainly borrowed from it quite liberally when writing Dracula. As an early forefather of vampire fiction, it undeniably has an important historical status, but it has to be admitted that Dracula is a much more complex and satisfying read, building on and developing elements of Carmilla into a far more interesting whole.
But Carmilla is not totally uninteresting. I suppose there are some spoilers ahead, but to be honest, it's not like the plot isn't extremely predictable, so ima go ahead and "ruin" it. The most entertaining thing, to me, about the novel is the fact that Carmilla is pretty forthcoming about her murderous intentions on the narrator. She keeps repeating that she loves her, and that this love will lead to the narrator's death. The narrator declares these odd speeches to be unintelligible, symptoms of some strange illness or general flights of fancy rather than openly expressed death threats. One could reflect, I suppose, on the way love and death are so intertwined as to make this confusion somewhat plausible - but it's really not. Even in the context of the story, Carmilla sounds completely loony tunes, and it's preposterous that the narrator is so blind.
The other vaguely interesting aspect is the unbelievable clumsiness of the denouement/explanation. After all this suspense and mystery, just when the novella starts planting some clues and making it seem like the family is finally gonna catch on to what's happening - a family friend whose daughter is dead shows up (admittedly, this has been prepped already in the opening, when we first heard about the daughter's death) and relates the story of his child's demise. Whadya know, it is virtually identical to the narrator's own story (ironically, it was the death of this daughter that led to Carmilla being welcomed into the narrator's home!). Mystery solved, next there is a rapid slaying and some pseudo-scientific explanation (couched in terms of an attempt to persuade a skeptical audience and deploying all kinds of scientific language) and boom, the end. 60 pages of build-up, 4 pages of climax and resolution. It might not be quite that extreme, but it's pretty close.
Like I said - makes you appreciate (and want to re-read) Dracula.