The family has more vehicles than we can shake a stick at, '95 Chevy 3/4ton conversion van, '95 F-150, '91 Miata, my wifes '05 Mustang we just bought and now my fortwo. We also have 3 motorcycles. The smart is my commuter, runabout and bad weather backup to my Ducati. We're getting rid of the van and one of the motorcycles now we have the smart and the Mustang.
I have used it for the last 11 months as my only car. It has handled all types of weather including heavy snow and ice. I replaced the tires with Snow Controls and have been very happy. I have had no problems with the car whatsoever and have had no reason to take it back to the dealership.
My only disappointment has been the fuel milage. I am not just talking about winter, but overall.
Obviously, with a family a Smart is unlikely to be an only car. Plus you can't go out with another couple (or even another friend, unless they fold well). So my Smart (due late summer) will be my commuter, a replacement for the used '98 Chevy Lumina that I bought off friends when they moved. My younger son will get that car (he graduates high school this year, and intends to go right into the workforce) and I will use the Smart for commuting to work (only 13 miles each way), for errands, and for local driving. For longer trips or more than two people, we'll use my wife's "fun" car--the Sebring hardtop convertible.
It is my only car. I drive it to work (47 miles everyday) and for fun (up to 1200 miles trip). Got great mpg: over 60 mpg when I am not in the hurry and down to 40 mpg when I am. So far drove about 16k miles in it without a single glitch. Accident avoidance ability of this car is nothing short of amazing.
My wife and I bought out "Scooter" last Valentine's Day (08). She drives it to work on a daily basis. It is the only vehicle we drive on the weekends any more. She has just over 23000 miles now. She loves it and would only replace it with another.
When I placed the order online for the smart, I had no idea that the Dodge Neon I had been driving since '96 would get totaled in a deer accident. That happened in November of 2007. We had to spend a young fortune on repairs for the Kia Sephia that my wife drives. It is now known as the Rolling Repair Bill. This is taking the place of the Neon, and is used 7 days a week. It is used for the daily commute, senior care and any other running around. It will be seeing a lot of Old Route 66 this year too!
I have an other car, but use my smart not only as a daily driver but for traveling too, it is very comfortable safe fun and great on gas, as a matter of fact I have taken it all the way to Puerto Vallarta Mexico, great on the two way winding roads of Mexico.
I work at a smart center and I would say many people are buying the car for a replacement for their main driver. I have even had one customer who purchased 3 cars to replace 3 of their big family cars.
Madeline (smart) is the 3rd car. We also have a 06 HHR (hubby's - the Hound mobile, as it transports the dogs to their babysitter's each day), an 07 Xterra (mine - the long winter trips up north, camping, kayak hauler, etc), and of course, Madeline (also mine). I drive 35 mi each way to work every day and Madeline will get me (hopefully) twice the mpgs as the X. Madeline just got plates the other day, so she hasn't had a chance to really go out on the road yet.