My 2002 S, purchased in April 2005 with 13,000 miles on it has only had 1 minor problem. The low horn malfunctioned. I purchased the Superloud Italian Air Horn from Griot's and had my dealer install it. They credited me 1/2 of the labor to install it as that is how long the factory horn was to install. Now when I honk the horn people are sure to notice!
Other than that issue, my Boxster has been trouble free.