It's saved me a few bucks here and there, nothing huge, at least not yet. As long as the items are appropriately marked as made in a NAFTA country, you're set. The only thing I've had to pay duty on in recent years from the States was my OEM headlight assemblies for the Audi; they were made in Germany and were quite expensive. The duty wasn't too bad, though. In any event, it was significantly cheaper than buying from the dealer, who wanted around double the price.
I've bought a few things from Rock Auto and the like when my usual suppliers can't get get it or quote me something ridiculous like a two week wait.