Most reviewers have gotten it right as far as the acting and style on this movie, so I won't add much as far as that goes.
If you watch this one more than once, it should be obvious that the story is not about two roommates, but one woman struggling with body dysmorphia.
The character's name is Jill, but she looks like Jennifer. The Jennifer personality may have been her original personality, or it may have been created as a defense mechanism to hide all the feelings of confidence and self worth, while loading up the other personality with guilt, regret, and self loathing. This was created during or after her marriage to an emotionally abusive partner who, as it is depicted in the extremely twisted and weird reality sex show sequence, would constantly tell her how disgusting and unattractive she was. This created Jill. The personality that binge eats.
This is made clear through several key scenes. The two scenes where Jill and Jennifer are both in the presence of someone else (waking up with Rob, and on the curb with the police), Jennifer goes unnoticed and does not speak or even move. She is not acknowledged by anyone but Jill.
Then, in the scene where Jill opens her front door and the woman snaps cell phone photos of her, the photos are shown later to actually be photos of Jennifer. Despite that the woman's name is actually Jill, Jill is the one who does not exist in physical form. The appearance of Jill is a mental manifestation of her body dysmorphia.
Then finally in the last scene, when Jill injects the talent agent with that nasty Draino stuff she concocted, she leans over, and once again, you can see in the reflection that it's Jennifer.