Ok. I had the price for the Cabrio version stuck in my head. The base is $11,590, which will still buy a nice used Corolla (1-3yo certified, etc). The Corolla will get the same or better gas mileage AND have room for 4 adults and luggage. Plus, the car and company have an established track record.
From 1996-2003, I drove a 1993 Toyota Paseo 5spd. I regularly got 30mpg in town and 40+ on the highway. It was pretty peppy, though no race car. My parents' 2007 Corolla gets low to mid 30s around town and low to mid 40s on the highway with AC and an automatic transmission. It costs more than the Smartcar Cabrio (just barely), but is a more versatile car.
The Smartcar would make sense to me if it cost less than $10k (with AC) or got 40/50 mileage.
About the only benefit I can see is the size will make it easier to drive in crowded cities, but all the ones I've seen have been out in the 'burbs (at least one had DC plates though).
Chris