I fitted some 35W 4300K units a couple of months ago. The ballasts came with a small metal fixing bar with a screw hole on it, to make mounting them safe and easy. The one for the nearside I attached to (what appears to be) the engine ECU, and on the drivers side there was a conveniently located screw. So my ballasts are screwed neatly and firmly into place, and the cables to the headlights are cable-tied neatly too. I'll put some photos up if anyone's interested.
But more importantly, my car went for MOT with the HIDs still fitted - and passed no problems, not even an advisory. I had checked (as well as I reasonably can) the beam pattern and it's straight and low with a sharp cutoff, so less risk of dazzling. I certainly haven't had any flashes from oncoming drivers either. The adjusters are set to _ _ rather than \__ which probably helps.
All in all, very pleased - much better light output, in a non-stupid colour, and no MOT issues either. As per the thread subject line, best £45 upgrade you can get.