JD Power ratings are worth the $1 they pay each respondent.
We have a 2003S which we bought new. The engine and transmission were replaced under warranty in separate incidences. The dealer service was excellent, and the car itself has otherwise been somewhat reliable (window regulators require occasional replacement, and the top motors were recently replaced, but I don't hold that against a 10-year convertible).
The dashboard AC mechanism was replaced under an extended warranty (Warranty Direct). Other work was done under extended warranty, but I do not recall the details at this time. However, I do recall that the cost of repairs performed under extended warranty overall exceeded the cost of the warranty. In preparation for an upgrade to a 2014 Boxster S, I solicited Warranty Direct for a quote. WD does not list the Boxster as a selection for coverage on their web page. Additional investigation revealed that WD ceased to offer extended coverage on Boxsters with the 987 model due to the excessive incidences of repair. An email to WD resulted in a reply that they would offer coverage on a 2014 model if I contacted them directly.
I do not consider a Boxster a very reliable vehicle. If I had not had to have had the engine and transmission replaced (unprecedented in my experience), I might consider it to be of average quality. Although it has never left me stranded, I would not be considering purchasing another if an extended warranty was not available, and if the dealer did not get every repair right the first time. Surprisingly, the most reliable vehicle we have ever had (which we still have) is a 1993 Jeep Wrangler. Almost nothing has gone wrong with it, with the exception of those repairs that resulted from my abuse or modifications.
Keith