My GF has one. Speedsilver edition, 82hp, Brabus wheels and some other Brabus bits here and there.
I've also driven other Smart Roadsters including the ones with paddle shifters (they're fun, but gear changes take a bit of time) and even the 101hp Brabus version (honestly, I couldn't feel any difference between this and the 82hp version). The engine size in all three hp versions of the Roadster are the same size, but I think all that changes is the size of the turbo, and in the Brabus version the turbo is just bigger (gives you a few tenth of a second faster times).
My GF really likes her 'Smartie', I think she even prefers it over my SLK 55 AMG because it's so 'cute'
What dani said is true. It's a light car, the seating position is quite nice (although the passenger usually ends up with his feet almost straight if they pull the seat back all the way). I'm about 1m80 (my GF's a bit shorter) and we both fit in fine.
It's not all that easy to get in or out of the car if you park in a tight spot (the door sill is high), but if you're not too old, you won't have too many problems
The roof is quite ok, I personally prefer the electric fabric roof more then the two piece one. This means that if you're doing a short trip and want the roof open, just hold down the button and it makes way. Then if you want additional 'open air' feeling, just remove the side bars and you're set. If you choose the two-piece roof thing, you'll only be able to experience the open air feeling if you remove them and leave them in your garage (or in the rear trunk of the coupe version where there's enough space to store them). I really suggest you use ear plugs if you drive on the highway with only the middle part of the roof open (the electric curtain part), because a LOT of air turbulence is created. But if you remove the side parts as well, then it surprisingly goes down to perfectly acceptable levels
In Switzerland, you can get a special Smart insurance which is only a few hundred punds ($500 or so) even for new drivers per year, and it consumes little gas (like 3 or 4 liters per hundred km's on the highway, especially with the lowest power version).
Dani also mentioned it being slower and more comfortable then the Elise. This also is true, but I suggest you get leather seats for the absolute best in comfort. Service is every 10 tousand kilometers, so it's a bit often, and costs about $200 (my GF just had it recently), but so far she's had zero problems.
The car seems to be holding up just fine and gets us a lot of fun, especially as it's very grippy and you can go around curves with a lot of speed.
But the gearbox is the biggest problem with this car. You could possibly end up in a nasty situation if other motorists don't see you due to your size. But the car holds up well in accidents (having MB standard safety requirements), and most people who mod the Smart say that a simple air filter swap really increases turbo responsiveness.
I think the fact that the car has very low power is the main reason why the slushbox takes time to swap gears. There just isn't enough movement created by the turbine like power distributor in the gearbox to get things moving inside fast enough...
I'd say that the advantages of the car being cheap to insure and maintain outweigh the disadvantages of the tranny. It might not be the fastest car under the sun, but it does mean that you won't risk getting fined for speeding (especially if you're a young driver, where these things can cost you an arm and a leg in the long run). Initially the car costs quite a lot for what it is, especially as the standard models have a lot of plastic and fabric inside, but the Brabus editions (honestly in my opinion too expensive for what it is) does use a lot of leather and aluminium bits.
The car is being discontinued, so you might be able to find a second hand one or even brand new for quite cheap. You don't necessarily need all the cherries, might as well keep it an economical, and fun car, because that's exactly what it is trying to be, and I think it succeeds pretty well. Some people loathe it, but it gets a lot of looks from passer-bys.
Always test drive one before buying it tho', maybe it's not the car for you, but I do know I'd rather drive a Smart Roadster then a Fiat Punto or Peugeot 206. Just a personal opinion.