I loved Let Me In
. I bought it as a blind-buy based solely on the trailer released prior to its theatrical release, and I couldn't have been more pleased. The two young leads are excellent and the movie is gorgeously shot. Michael Giacchino's score was also very haunting as well.
As for the ending, I liked it. I didn't see it as a "cliffhanger" that sets up a theoretical sequel, but more as an ending that allows the viewer to simply wonder how events will unfold.
I certainly wouldn't call Abby a villain. She's more a tragic victim of her own nature, as is her caretaker in a strange way (played wonderfully by Richard Jenkins, btw).