These cars have a reputation for being unreliable, however, if you look after them not that much goes wrong, other than normal wear and tear.
When I purchased the car the cylinder head gasket had gone! This was down the fact that the correct servicing schedule had not been maintained; the head bolts need to be tightened down at every service as they tend to loosen due to poor design, thus ruining the gasket. In Europe a 700cc Cinquecento is available (using the same engine) with an updated head, during one of my regular trips to Spain, I obtained one of these cylinder heads with the new specification gasket and I have not had any problems since!
The car uses quite a few rear tyres, I tend to get 15,000 miles out of them.
The driver's door handle broke on one occasion and I spent about a month getting in and out through the passenger door. The handle is the same as is fitted to the Cinquecento, so eventually I found one in a scrap yard and fixed it.
I have been through three clutches! And countless hoses for the cooling system, particularly the one that runs to the front of the car.