This movie takes the step of answering what might happen if you ever asked that pretty girl on the train to get off at your stop. Jesse (Ethan Hawke) and Celine (Julie Delpy) do just that, and spend a day together in Vienna before Jesse flies home to America. It's clear that they are attracted to each other, and not just because they're both Hollywood good looking. They talk (and talk and talk) incessantly, about world views, about their emotions, about life in general - if that's not your thing, you should avoid this film.
But it very much captures the spirit of being young and idealistic and willing to take a huge leap into love and life. While it's a flawed film (they seem to still know very little about each other at the films end, for instance) it also doesn't rush for a typical Hollywood ending and does a great job of showing what it's like to be young.