I see two or three a week, but that may be a function of the places I spend my time. They strike me as tippy and underpowered. I could see using one in the city, but I wouldn't want to take one from Portland to Seattle, or to the mountains.
I've driven a smart a few times, both on the motorway and in the city, and I have to admit that it's really quite pleasant to drive, in town anyway.
In town, it feels really, really nimble and quick. It accelerates quickly, it parks easily, it fits into small gaps and spaces, it can turn on a dime, and generally it just feels good.
On the motorway, it's ok until 75mph or so (speed limit being 70mph here). Above that it start to feel very light on the road... 85mph was the highest I dared to take it, and it felt like every gust of wind might knock me off the road. Around 70mph, though, it felt small but steady.
On the whole, I think it's a really nice car, particularly for those who like a modern gadget feel. The Smart feels like an iPhone car, if you see what I mean. You wouldn't do your excel spreadsheets on it, but it's damn nice when you're out and about in the city.
Seen a few, even thought about getting one, but for 2K more I got a used 3 year old car that has a bigger trunk, back seats, much better gear box (manual 6-speed) and is overall much nicer.