There are pro's and con's.
The only kind that make sense are those that have HID lo-beam and HID Hi-beam - the HID is a single burner that is physically relocated inside a housing by an electromagnet.
The cheap ones have a HID DIP and rubbishy (generally low wattage) Halogen MAIN. This also means the HID is switching on/off - they fail fast if you do so.
I've converted all sorts of vehicles to HID and eventually took them all off. Hiding the ruddy driver boxes is a sod on bikes. If you were closer - I'd invite you over to fit one.
THERE IS A PROBLEM with the Triumph light units too. The GOOD versions of the HID conversions will not fit through the bulb hole. Got the t-shirt.
Also - my experience is that the wattage is a bit closer to 70% of Halogen.
The future WILL BE LED. Proper units are still too bl**dy expensive. But if you're up for R+D work there are LED torches out there with outputs 8 times the power of a Halogen bulb (twice and a bit the power of HID)
A Halogen puts out approx 1000-1500lumens. HID approx 3500lumens. I've seen 10,000lumen LED torches.
Converted some "Triumph" auxiliary lights to LED using 1000lumen projectors.
http://www.youtube.com/watch?v=VF9JGnu9B_8