The coil works good it would be better with a battery and full on charging system but that would be a lot of work and weight but it would be worth it if you ask me. the light goes bright and dim with engine rpm it would not do this with a battery.Yes I did have a problem with it if the puck tuoches the coil at all it will destroy the coil it is inportant to check the crank run out.I also put .002 shim stock to wrap the center bolt where it goes threw the puck this helped to perfectly center the puck make sure you get the coil lined up good or you will find yourself out in b.f.e. with no light.
As far as the economy goes I try not to pay much attention to it. The world is ran by idiot's and that's out of my control. Gas now costs me almost as much as your beer and its getting to the point that I almost can't afford to drive to work.
I already have the battery anyway as I kind of like stuff like indicators and good lights, it makes the bike look better so the police are less likely to stop me and ask if my exhaust is legal (ahem) for the road.
A fairly simple to make wiring diagram and tutorial can be found
HERE and after making my own a while ago and not being quite happy with some of it I have just bought some GSXR left side switchgear to make a better job this time.
I have a length of seven core trailer cable as well so this time it will be a lot neater.
I also don't like lights that are dependant engine revs cos oncoming drivers think I am flashing main beam at them.
Thanks for your informative reply.
Economy, Don't get me started on petrol prices...
Its 89p per litre over here which works out at about $6.73 per US gallon if my maths is right
Edit.
After buying the GSXR switchgear like shown in the link and having a good look at it it will need to be modified a fair bit to work with the wiring diagram they show, I only want to be able to use fairly standard switchgear cos that means if I damage it its simple and quick to replace so will modify the wiring harness instead, this might well mean that I will have the only KX500 in existance with fully working hazzard lights