It's not bad. They are reasonably reliable, at least in the gasoline engine form (the original diesels were trouble, with one of the two original makes of engine described by enthusiasts as having a 50K km or 30k mile life expectancy). The most common cry for help I hear is when the driver is faced with a dead battery and has no obvious way to get in. All things considered, that's not bad.
On the plus side, the SMART Fortwo is very compact. On the minus side, it is very compact. It is a few thousand dollars cheaper than a Prius but has greatly limited passenger and cargo space and much poorer fuel economy. If those are not an important issue to you (as would be true for a single person or a couple who has no children and drives mostly in town), SMART could be a good choice.