Glare. Don't let anyone tell you there isn't, because there is. I had a PNP kit in there, and I took it out. Sure it improved on the lighting, but now days as other cars that have PNP HID kits approach me from the opposite direction, I know what mine looked like before I took them out.
A halogen reflector was not designed for an HID bulb. Period. Retrofit an HID projector into the stock or an AM headlight or IMO don't do it at all.