Mine has done pretty well in Georgia, though I have to admit it pains me at times to leave it out - unfortunately I don't have a garage at home. Just like Atomicblue99 mentioned I use a California Pop Top more often than not (again - it leaves some white fibers on the canvas, but that's better than constant sun/water exposure and comes off during cleanings), and I'm in the market for a better car cover. Preferably one that is machine-washable because otherwise it's just impossible to maintain the cover for a long period.
You just need to make sure you keep the car waxed and the top clean & protected. I use this stuff for the top:
Amazon.com: 303 Products 30520 Convertible Fabric Top Cleaning and Care Kit: Automotive
In my experience it works better than RaggTopp, and I paint most of the protectant on with a large brush. If you plan on having your Boxster for a while and it stays outside a lot you might as well buy the protectant in the gallon-sized container.