I drove a friends smart car a few months ago. He had modified it a bit (K&N and remap) It was good and at no time did I feel unsafe in it. I only took it up to about 70 kmph as we were in town. The sequential shifter is pretty cool but in auto mode there is a huge delay between shifts. He recommends leasing it and then dumping it because only MB can work on them which is some big $$$. I've heard that he doesn't do all the maintenance that he should though so that should be taken into consideration
I believe that you can get a chip for the smart to boost it up from 40 ish HP to around 70. Should be really peppy then. Cheers Dan