They are awesome, if done right, mine usualyl runs one HID and one halogen and the HID by itself is far brighter and more effective than 2 Osram Nightbreakers (which is as good as you'll get the SV headlights with halogens)
It's not legal... But, as long as you don't have them set up dangerously (which is very common- these things are bright, and not designed for use in a halogen style reflector) and you use a low-temp one (ie, not too blue in colour) then you're very unlikely to ever have a problem. I've driven along behind traffic cars and not been stopped
