Just got back from seeing Slevin - well actually got in an hour ago.
Both me and the fiancee really enjoyed it - top film.
Well made, excellent cast and although I'm sure you'll have some of the twists worked out I doubt you'll have everything before it is revealed.
Definite "Buy on DVD" once it gets released as I want this film in my collection.
Side Note:
Recently I've found my cinema experiences pretty bad - people talking, rustling, etc throughout the film.
This was the first Wednesday after Slevin's release (this means first day for the Orange Wednesday crowd).
The screen was quite full for a film starting at 9:20pm on a Wednesday but the experience was excellent.
From the moment the film started everyone just shut up and I didn't hear a sound out of place for the whole film.
It takes quite a good film to keep a whole screen quiet throughout!