Google it and you'll find 1,000,000,000,000 threads on it.
First - HID's in halogen headlight housings = rice...glare...blinding....and terrible light output
Secondly - There is a reason as to why HIDs in factory vehicles are ALWAYS equipped with projector type housings versus halogens reflective housings. The actual light output in terms of light are different between hid/halogen.
Third - Any sort of "cool" blue, purple, or green HID's actually put out less lumens than standard halogens.
The reasons continue but I think thats a good start
First - HID's in halogen headlight housings = rice...glare...blinding....and terrible light output
Secondly - There is a reason as to why HIDs in factory vehicles are ALWAYS equipped with projector type housings versus halogens reflective housings. The actual light output in terms of light are different between hid/halogen.
Third - Any sort of "cool" blue, purple, or green HID's actually put out less lumens than standard halogens.
The reasons continue but I think thats a good start