We have gone through some pretty windy days with the smart, and it's not too bad. It reminds me of when I was driving a '66 Volkswagen Beetle. You find yourself with both hands on the wheel when there is a good gust of wind. Then one of those real strong gusts can sort of catch you off guard, but it's not too bad.
On the interstates, passing a truck isn't bad either. To be honest, it holds to the pavement better than the old Beetle did. Just allow plenty of distance for acceleration when entering a road off of an exit ramp.
I guess it's the age showing through on me, but I have gotten to where I would rather travel on an old US or state highway rather than the interstate. Not because of the smart, but the traffic is so much lighter, plus you see the small towns. Time doesn't always allow for the older roads, but they are nice when it does.