They're hard to judge quickly, so here it is. They increase the contrast of objects at night, but at the same time slightly darken everything around them. They reduce glare from oncoming headlights. When on, HID bulbs look more like halogen headlights without them, and halogen headlights look more like old pre-halogen headlights without them. In other words, you no longer have to look away or turn your head to see HIDs and they're still bright, but you can see through them and look straight into halogen lights and see way beyond without discomfort. However, they also work to dim your headlights on dark roads so you can't see everything as well as you used to. This is kind of a compromise and depends on what you want from them. For me it slows me down on dark back roads, which is probably a good thing, and it reduces glare, which allows me to see past oncoming cars instead of looking away briefly, which is also safer. In general I feel better with them than without them. They work best in the city or on country roads where there is ambient light such as billboards, houses, road and traffic lights, etc. On country roads in total darkness you only benefit from reduced glare from the occasional oncoming vehicle, but you can no longer see yourself as far or as far as you could before, despite the area you can see well having higher contrast.