I've had a few convertibles in the past and the smart fortwo seems to be better than average. That said, I've haven’t lived in the real cold stuff as our lows rarely go below the lower teens during clear winter nights. I also keep my cabrio in the garage.
The only thing I would keep an eye on is where the convertible top meets the top frame of the windshield. I always wipe my seals clean after I wash my fortwo to ensure I have a clean watertight seat. Over a period of time, trapped debris can deform the seals just enough to let water in. If you wash your cabrio in cold winter weather due to salt (understandably so), be careful if the water freezes and you try to open the top to clean the seals. Either it’ll stick and you won’t be able to open it, or you could risk tearing a little bit of the rubber seal off. Since I’m no expert in very cold weather, I would probably keep the top closed after washing until you’re in above freezing temperatures for an extended period of time. Then I would open it to wipe the seals from dirt and salt.
I don’t know if this helps or not.
-Tom L.