I've done HID retrofits on three cars. I bought cheapo HID kits from Amazon for all 3 (One was $50, one was $60, and one was $75). I used the stock headlight housing.
2007 Scion TC (9006 lamps - Xentec) - Looks good. I get people that flash their brights at me maybe once a month if that. Much brighter, and the beam spread looks the same. Been using the same ballast/lamps for about 6 years now)
2007 Toyota Highlander (9006 lamps - Kensun) - Looks good, much brighter, and the ballasts are nice and small. A bit more glare-y, but partially because i need to do headlight lens correction. When they were clearer, they looked better, and had less people flashing their brights at me. Been in use with original bulbs/ballasts for about 2 years.
2004 Grand Marquis (9007 Bi-Xenon - Innovited) - Brighter than stock halogen, but an AWFUL and i mean AWFUL beam spread - inconsistent. When I have a chance, I think im going to swap to LED to try it out.