My next door neighbour had one (smart for four) for a couple of years, it was in the garage being fixed more than it was in his garage, most of the problems seem to have centred round the computer saying that it was broken, rather than the fact that it was broken.
He got so peed off with calling out mechanics to look at it that he bought a Vauxhall of some sort, he's never had a problem with that, had it for 3 years.
edit. his wife is on her second Smart for two, she loves it!