I know some people who have fitted similar kits to their cars, and I have toyed with the idea for some time.
The problem is that these kits are not road legal.... I know the chances of getting caught are slim, but who knows what would happen if you had a harsh insurance claim and such a kit was found fitted...
The kits themselves are easy to install. I think they take a feed from the battery for the Ballasts, the control wires should then plug directly into the existing vehicle wires/ plugs to actually control the kit. You will also need to fit the ballasts somewhere secure and dry, but with all the room under the Z4 bonnet it should be fine.
Reports from people I know is that they make a massive difference, lovely crisp white light. Have not heard of anyone failing their MOT's either.
there was huge long thread going on this for the last week or so on this very subject with some good input from CJ.
Basically it boiled down to any kits in the UK are illegal and pooh. If you want to convert to HID/Xenon properly you are looking at massive amounts of money (£1500+)
is it 'start a thread night' tonight ryukin, you've got loads
edit: search is your friend - http://www.z4-forum.com/forum/viewtopic.php?f=2&t=14305&hilit=halogen
Thanks for the replies. It sounds like what I want, but also sounds more hassle than it worth. Maybe I should have thought about these things when I bought the car!
Had a Xenon conversion kit on my Mazda 3 - possible as it had projector lenses. I made sure they were well aligned, and kept the headlights clean. Never had any problems and night driving was so much better.
Get a decent kit, install it well and I'll think you'll be happy. Not legal I know but get it set up right and you should be OK.