I did my first bit of highway driving with the smart. My route was from Houston to San Antonio (3 hrs), then to Austin (1.5 hrs) then back to Houston (2.5).
On the first leg the driving was a snap. "What's the problem?" I asked myself. Then a cold front blew through the area bringing much higher winds. The drive home was not easy with the 20 mph gusts. Tractor-trailer rigs didn't cause a problem, it was the cross winds that were a bit scary. I think you would get that in any car, however.
I have not yet calculated mileage. The car has about 1200 miles on it now.