I ran them for a couple of years. They were definitely brighter than the stock C5 bulbs. But A candle would be brighter than the stock bulbs.
The problem is the factory housings are not designed for an HID bulb. you need a proper light cannon. The beam becomes very messy and also shorter than the halogen bulbs.
I do quite a bit of night driving and I found they gave me a headache after a while.
I kept them in the fogs as this is a good light cannon.
Also the offshore ballasts are always hit and miss. Could count the number of times on one hand that all four lights worked at the same time.
I switched to the high end Silverstars and am pleased with the results.
I just threw away the light harness and bulbs, while purging for next weeks move.
Ask the seller for his warranty policy in writing before you buy.
Some guys love them, they were just not the right fit for me.
There is a vendor that makes a proper light housing conversion kit for the C5, but its close to $800.00 . All reports say they are worth the money.