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.
Edit -- on second thought, I guess BMW could be it. They are still independent and don't produce anything cheaper than the Mini.