I saw the Smart in Italy years ago, and at the earliest opportunity here in the U.S. drove one. Admittedly it was early-generation (for the U.S.), but it had years of market time in the EU.
In a word, it's a novelty whose charm wears off very quickly. The ride is uncomfortable, it doesn't hold enough, it doesn't generate a feeling of security, and it's very costly.
There is no reason to keep miles off of our '04 Prius. There are plenty of them now in package and document delivery, taxi service, etc. with 300k miles on them and statistically they're holding up better than their non-hybrid counterparts. I think it's due to (1) Toyota spent extra effort on quality for this innovation and (2) the hybrid system "unloads" the engine during initial acceleration, when wear and stress are high.
But if you feel compelled to pay registration and insurance on another car to reduce your Prius mileage, buy the Honda Fit. My daughter's Fit gets 36mpg on regular gas, and four people fit comfortably with space for small baggage. In fact, with the Fit's remarkable folding seat system, two people plus two bicycles plus camping gear can be accommodated. Try that in a Smart!
Never, never compare the Smart to the Prius. They're in different vehicle categories.
Ike