To keep your recently buffed/restored plastic lenses from yellowing again you need to apply a coating with UV protection. Some headlight kits do come with a coating or use something like Optimum Opti-Coat.
The glass lenses on my 02's headlights didn't fade,but it has a lot of small rock chips. Before I install the new headlights I'm going to apply a clear film for impact protection. My replacement lights are glass but I would still install the film if they were plastic,the name brand ones also have UV protection which is a plus for plastic lenses.
If anyone else buys those projector headlights and does what [MENTION=74071]Brilliant[/MENTION] did (swap existing halogen projectors with TRS Mini H1 projectors), I would hold off buying the Mini H1 projectors until January when TRS releases the 7th generation design, they'll have a beam pattern wider than one of the best factory HID projectors ever made (Honda S2000)