The only legal way to fit aftermarket HID's is to buy the entire HID light unit (bulbs, ballasts, housing, optics the lot), a headlamp washer system, and auto levelling. The light unit must have been designed from scratch for HID usage and not halogen, and fully 'e' marked with lots of other legislation and tests passed. Headlamp washers + self levelling is easy enough, but no company has designed and supplies the HID compliant housing.
As above aftermarket HID's will be picked up on the 2012 onwards MOT tests with other VOSA changes, and will result in a fail. HID's would also have to be insured, which insurers should refuse as the only way to do it currently is illegally. A police car will also have the right to pull you for it, and possibly fine you.
There are steps you can take to make a HID conversion safer. Stay below the 6000 Kelvin Mark. Above 6000K there is a blue tint in the light, this causes a glare that eyes are VERY sensitive too, when exposed to blue wavelengths of light the retina's open up, letting more light in, this gives the receiver a perception of brighter light. 5000K is a nice cold white, without blue and therefore is a much safer colour, although it's actually brighter in lumens.
I'd also advise 35w in dipped beams rather than 55w, they're a bit dimmer and aggravate eyes less. Full beam/Fog lights shouldn't ever hit people in the eyes so do what you want here.
Make sure you have the projector housing! (chrome headlamps, standard on Zetec and above) Although it's still not completely "safe" it is safer than using reflector dishes.
Fit and adjust them properly.
Keep them dipped low enough to avoid the light entering windscreens of oncoming traffic.
At the end of the day though it's up to you, I thought about it, but felt it was too much hassle. I'm neither for or against really, I don't think they're too bad when done properly, and the lights sit pretty low on the fiesta anyway. It's HID's on 4x4's we should be complaining about!
http://store.candlep.../trabhidki.html - Independent review of HID conversion kits and warnings from the largest online light community. Very informative.
http://www2.dft.gov....dheadlamps.html - Department for Transports view on aftermarket HID conversion kits & the law.
http://www.fordowner...12-mot-changes/ - Some of the 2012 MOT changes discussed.
Good luck, hope this helps, sorry it's a long winded post.
you think HID's are dangerous? In ten year's we'll be using lasers as headlamps (I'm not joking).