I used a roadster for a couple of days whilst on holiday 2 years ago.
As already mentioned, the gearbox is really not suited for spirited driving, but frankly that's about the only bad thing I can say about the car, apart from the obvious point that it's got bugger all space (an MX5 does have more boot space).
It was fun, great steering, acceptable ride, the one I had didn't leak (drove through a rainstorm for a while), handled very well and had far more grip than go. It felt like I was travelling at 100mph when the speedo was registering 100kph, which can be a good thing. If you want fun at the expense of motorway cruising ability, it's an interesting option.
The biggest problem with the smart is that the MX5 does exist, and frankly it's a better car.