I think they are interesting enough, and enough car for many people's daily routine. If I lived in a very congested, urban city, I suppose a Smart could be an okay second car.

But I don't see a reason to pick a Smart over any number of other small cars.
- It doesn't get particularly awesome gas mileage
- In most US cities, the legal parking spaces are all standard-sized, so being super-small doesn't get you any additional parking choices
- It is not particularly cheap
- When you do need to carry >2 people, or more stuff, or take a long drive, it doesn't work so well

Although I have thought of one situation in which I'd buy a Smart for sure. If I lived in San Francisco, and right in front of my house there was a snippet of sidewalk between two driveways, that was too small for any normal car to park in, but just large enough to be my private parking spot - then, its a no-brainer.