I found the flashback/flashforward jigsaw not at all confusing. In fact, it was very effective and produced just the right balance between revelation and suspense. The photography was lush and sensual, grading towards gory and horrifying towards the very bloody final scenes (which also eased this film into Cat 3 - there's no nudity, and only one indistinct sex scene). And the ending is brilliant - as simple as it is stunningly elegant.
A few warnings. This film contains frequent drug use, which I found excessive, even though it was fairly important to the story. When the means of chopping up the body is eventually shown, not a lot is left to the imagination. Lastly, a "don't try this at home" warning : To all you nice guys out there, don't try to imitate Billy's character, even though it's a technique which clearly gets the girls !
Overall : Not a pleasant or feelgood film by any means, but an effective eroto-gory chiller nonetheless.