I have used them in cars and bikes with no issues other than being suseptable to any dirt or engine running rich as the tips are so fine - If your car is running fine there will be no issues but problems will manifest themselves quicker through dirty plugs, which may not be a bad thing as you identify issues earlier.
The one plug I did find has made a noticable difference to fuel consumption and engine feel over standard plugs are the Bosch super 4's - not tried the platinum version yet though