This is probably going to depend on which source you access since the opinions are many and varied. Most of the reports and sources I have accessed seem to think NAFTA has been a detriment to the US economy in terms of trade with other countries.
The answer to this is not as easy to determine as one might think. Various scholars come up with different answers. One answer is that given by the author of the paper at the stlouisfed.org link below. He argues that NAFTA has increased trade among the various parts of North America and has, in general, decreased trade between North America and other regions.
For example, the author (Howard Wall) asserts that trade between Canada and the US and Canada and Mexico has increased due to NAFTA. He argues that Canada exported 29% more to the US due to NAFTA than it had before the agreement was passed. He also argues that Canada decreased its trade with both Europe and Asia while Mexico decreased its trade with Europe but increased its trade with Mexico.
So, by one evaluation at least, NAFTA has increased trade within North America and decreased trade between North America and other regions of the world.