I should probably see this one again, to sort out all the details, but a first viewing was enough to have me hooked on the story and characters, and left me thinking about them for days afterwards.
As a thriller it is superb, drawing you in slowly, building tension by what is left unsaid, by limiting your knowledge of the characters, and also by making them both thoroughly likable and somewhat detestable. A long, musical scene where the immensly attractive if somewhat sinister Jack rescues a cat from a ledge is a perfect example--he's irresistable to everyone, including the viewer, but we also see the way he stares down the cat and don't quite trust him. Colin Firth's lonely, film-obsessed and alienated loser Adrian is fascinating in his own right.
The heart of the movie lies in the many difficult relationships-Adrian's dislike of the other tenants, their adoration of Jack, and the marvellous, tense, sexually charged relationship between the two, sort of Withnail and I meets Vincent and Theo, meets The Talented Mr. Ripley.
If I have a complaint, it's that all this is carried a little too far, and I think it might have ended a bit better with that plunge from the balcony. Still, that's only a small quibble for an original and daring film like this.