The key to this movie is the viewer's ability to overlook technicalities and appreciate the story itself.
Keaton is superb as sociopathic "Carter Hayes". Initially, he is charming and convincing as a well-heeled business guy. He talks his way into Patty and Drake's rental apartment and then takes it over, sans credit check or invitation by a series of lies and mild trickery. Once he's in, he's not getting out either.
There are some excellent camera shots, especially one of Griffith (or Modine, can't recall which) in their garage, while the shape of Keaton appears from the shadows behind, staring blankly at the back of the protagonist.
The power of the story lies in the long setup of frustration. Griffith and Modine represent the "everyman" middle class couple who don't really mean any harm and have a pretty good life together. When their life is more or less ruined by Keaton, completely unprovoked, and they are continually tormented and screwed over by the police and legal system, the viewer gets a blood lust for "an eye for and eye" revenge.
The subsequent screwjob that Griffith unleashes on Keaton is thoroughly satisfying.
There are some weak points in the plot.
When Griffith is on her revenge quest, she walks up to the front desk at the hotel where Keaton (who has, by now, assumed the identity of Modine complete with personal documents and social security card) and asks out of the blue if there is a Drake Goodman staying there. The desk attendant readily complies. It seems extremely unlikely that any hotel, particularly one that caters to the obviously rich, would give out information to someone just in off the street. The weak explanation given is that she claims to be returning Keaton's wallet, however it seems like they would want more information and at least check the ID in the wallet.
Also during the revenge sequence, Griffith gains access to Keaton's hotel room simply by claiming to be his wife and asking the cleaning woman to let her in. The cleaning woman readily complies, even though she has never seen this woman before in her life. A little later, when Griffith finds documents in Keaton's room that tell her that Keaton somehow obtained all of her's and Modine's personal information and has completely assumed Modine's identity, she throws a major fit screaming and throwing items around the room. The cleaning woman who unlocked the door for her knocks and mildly asks, "Is everything okay?"
However, if you can overlook things like this, Pacific Heights is a great movie.