An excellent film. In fact The Conversation is so good that it helps to make even other movies seem better. In Enemy of the State Gene Hackman plays Brill, an aging, paranoid, surveillance expert. It doesn't take too much effort to look at this character as Harry Caul 20 years later, especially after the events that occur at the end of The Conversation. One can see Harry pulling even deeper into his shell and becoming more obsessed with those who might be out to get him, eventually going under the moniker "Brill" and building his hack-proof jar. This idea manages to add a little bit of depth to what would otherwise be a straight up action flick.
This scenario was even acknowledged in Enemy of the State. Both Harry and Brill have workshops in identical warehouses, one of the bad guys in Enemy of the State wears the same clear raincoat that Harry does in The Conversation, and there is also a scene where a picture of Brill as a younger man is shown, this picture is actually one of Harry Caul.