Once you've experienced HID's there's just no going back to halogen.
Tim makes a very valid point about big spotties reducing air flow to the radiator. But a small pair of HID lights will put out more light than a big pair of halogens. In my case I have a small pair of Narva ultima 175's converted to HID (55w) on my van that put out more light than the big Hella Rallye 2000 halogens on my fourby. The Hella's will be getting converted to HID soon.

Tony