The office of the U.S. Trade Representative has a great fact sheet (PDF) on NAFTA. Here's a good one:

Myth #2: NAFTA has cost the U.S. jobs.

Fact: U.S. employment rose from 110.8 million people in 1993 to 137.6 million in 2007, an increase of 24 percent. The average unemployment rate was 5.1 percent in the period 1994-2007, compared to 7.1 percent during the period 1980-1993.