Earlier quoted context omitted.
In my experience, Google Maps' Transit coverage is far far greater than "a handful of U.S. cities". It's worked everywhere I've been in Japan, in Singapore, Hong Kong, Sweden, London, Athens... The beauty of Google's Transit compared to apps maintained by the transit companies themselves is that it supports cross-company travel.
Not Paris, not Rome.
Found the list of cities. It's massive. It's true no Paris or Rome, but many other cities in France and Italy. http://www.google.com/intl/en/landing/transit/text.html#eu