I agree with everyone above that says it's a personal choice but we love it. I've put on 3x as many miles on my smart than on my Explorer. I'm not selling the SUV, which gets 10MPG city. I just drive the smart to/from work, and when we do our biweekly shopping, the Explorer fits 10x as much in the back.
I've owned it for 18 months and still get looks, people pointing, smiling and laughing. Kids love the car, and you'll get a LOT of attention in it. Less now than a year ago, but people are getting used to seeing them now. The car's so much fun to drive, I'd keep it even if Chevy made a Cobalt that got 100MPG. Especially since I drove a Cobalt 600 miles to pick up my smart, and was far more comfortable driving 600 miles in the smart.
Don't worry about the naysayers - safety is top notch. 35,000+ smarts have been on the US roads for almost 2 years and I think so far we've only had two fatal accidents. Its safety systems have helped drivers avoid wrecks, and the inside is in fact designed to minimize injury to the people inside. The little buggy's super nimble and I've avoided potholes and road debris with amazing agility.