I had them in mine and got on well with them, had them in for probably a year and had no problem. In reality from cold they take 5 seconds to warm up, and when you're on/off with them they take maybe 2 seconds. For brightness, nothing beats them, they really were quite awesome.
However, when I put HID's in the A6 I didn't do the mains, principally because I missed the instant light of a halogen bulb. I missed being able to fire the mains on for a second just to glimpse the road between oncoming cars etc - with HID's you can't do that. I have just put a set of Philips Xtreme Vision in the mains, they come recommended, but in reality after the HID's they're like yellow candles!