Carmilla starts off strongly, but if you judge by the amount of times I put it down and forgot about it, didn't stay interesting throughout, this is probably because the fact that Carmilla is a vampire (and the villain) is something you know before going in and the prose alone is not entirely enough to satisfy me as a reader. That does not mean I didn't enjoy it, but that any element of surprise is gone and it can get a bit tedious when they're all stumbling about not quite understanding what is going on (which is a thing that happen in modern vampire (or other supernatural/mythological/whatever creature) stories too, and I just want to ask them if they've never seen a movie or read a book before while throttling them).
I quite liked the setting, a lonely Schloss with a lonley girl in it, and then the surprise appearance of another guest, hurled into their lives by a mother who dares not travel on with her fragile daughter after a convenient carriage accident on the road outside the castle. The guest is the languid and beautiful Carmilla, who instantly develops a close relationship with the castle's inhabitants, the daughter, Laura, in particular.
Carmilla craves closeness and intimacy, along with the blood. Laura and Carmilla talk, laugh, walk arm in arm, hug and exchange kisses (but not on the mouth I think, you Le Fanu was a tease!). The latter making Laura a little uncomfortable, but she adores Carmilla. "I wonder if you feel as strangely drawn towards me as I do to you; I have never had a friend -- shall I find one now?" Carmilla says to Laura while telling her that she has felt that they've known each other forever, and Laura feels similarly, "but there was also something for repulsion" she explains, but the attraction and fascination were stronger, despite Carmillas weird behaviour and sleepwalking. While this is described as a friendship the romantic undertones really aren't very covert, you don't have to try very hard if that's what you are looking for here (and many have). And this relationship between a predator and it's prey is what makes this book interesting, and is probably why a lot of people read it today (in combination with an interest in the vampire's earlier literary days).
Carmilla is secretive, mysterious and beautiful, and of course, a vampire, the monster and villain of the story, but she bats her eyelashes and is, or pretends to be, exhausted and fragile and everyone loves her. She's a monster in disguise as a living doll. Carmilla and Laura mirror each other in many ways, both being (seemingly) young, lonely women. Both vampire and human crave a connection, but where Laura wants friendships, Carmilla is mostly hungry. There developes and intimacy and a friendship between them almost instantly (but not in an "instalove" fashion), because they're the only girls there. Laura has recently been deprived of a visit from another girl she had looked forward to seeing and is happy to have a replacement, and Carmilla is a delight.
The vampire is the typical one, to kill it you must stake it, chop it's head off and burn both that and the body in order to be sure it won't come back. It can vanish from sight (becomes air/mist?), and it kills its victims by draining them of blood (and life) slowly. One human = many meals. Carmilla is found in the end, sleeping in a coffin full of blood, her pulse and breathing faint, but present. It is also stated that one might become a vampire by suicide, "under certain circumstances" (whatever those are. Suicide is a sin though right? Religion is important here, because if you've attracted a vampire you don't need a doctor, but a priest), and once you have one vampire you'll have more, as it's nature is to procreate. Vampires are mostly mindless, but Carmilla is a bit of a different story and so forth. I wish some things were better explained (like the circumstances one would require to become one while killing oneself). The expert that comes to the rescue barley get to explain himself before they've found the grave containing Carmilla and beheaded her, the ending is a little rushed, but of course it could easily have been completely boring after Carmilla is found out if this wasn't the case.