Have been in quite a few in England, my Step-Daughter has one (her second in fact), her husband had one, ex-boyfriend had one ... etc. I was very put off by them at first but not any more after viewing some safety video's, and riding in them as well (including on the dreaded M25 racetrack to Heathrow!!). The "egg" that surrounds the car is actually quite protective and the lightness of the car makes them "bounce" rather than just "crumple".
That said, I think they're fantastic CITY cars, I would not however want one if I did all Interstate driving because of the heavy truck traffic. With the big Semi's it's not so much that I think I'll get hurt more in a Smart than I would in another vehicle, cause no matter what your driving when a Semi decides to hit you - your toast. What I am worried about is that the car is so darn small they're not easy to be seen by larger trucks. Trucks are not used to looking that low to the ground for other vehicles :)
But how much "stuff" will you need to haul around town? If more than a few small grocery bags, you will be looking for additional space. Smart cars are not made for hauling much more than the oxygen that the driver requires to remain lucid.
If your primary focus is fuel efficiency, there are more fule efficient cars out there that will have more space as well ... Honda Insight, Toyota Prius, Ford Fusion, Honda Civic Hybrid. Not 100% sure how each of these stacks up in purchase price compared to the Smart though (suspect the Prius at least is pricier than the Smart). Hubby and I test drove the Insight and the Prius and we didn't like them - both for the same reason - reduced rear visibility (they both have this bar across the back split rear window - bothersome). And of the course the Nissan Leaf which is all electric. I personally have no use for an all electric because of the limited range before you need a re-charge.
That said ... I think the Smart are fantastic little cars built by one of the most reputable automobile makers in the world (Mercedes Benz), and designed by one of the most innovative companies of the 20th century (Swatch).