The initial idea: young woman befriends much older woman, looked like it might have some legs to it, especially with the money angle to it too. However, from the start the movie seemed a bit unoriginal - Harold and Maude sprang immediately to mind - and implausible. Some of the stuff that happens is just plain weird and contrived.
Weaved in and out of this theme is Jane's (the young woman) other life, with her permanently-high flatmate, other friends and job. This varies from quite funny at times (the flatmate and friends) to downright gratuitous (the R-rated scenes). The porn industry stuff was essentially just padding and titillation. It cheapened the movie, to a very large extent.
Ending is interesting and does tie the movie together, but isn't that much of a shock or twist.