Absolutely, Drupal 7 is ready for big sites,Eventhoughsites; even though many modules are in beta, the essential ones are stable, and Youyou can start building with Drupal 7.
On otherhand Thereother hand, there is no need to hurry up, since Drupal 8 will be, most probably release after 2 years from now, (sourcereleased between 2 years). for
For your reference, read this article alsotoo.