the 4watt MR16's are amazing
trickiest part i find is mounting them. very hard to find good holders that wont fall apart. the convenience of just plugging into 12v and not worrying about polarity helps. i had one magneto that worked with them on the HT, but no others since.
$4 on fleabay i was finding them...
on standard 5mm leds, 180 to 220 R for the resisitor at 12v. 180R more bright, 220R more suited to auto thats really 14.4 ish volts.
for plain usability, try mounting lights as low as possible. personally, at least, with helmet mounted LEDs, i cant distinguish shadows/depth, therefore speed. potholes, or obstacles. everything goes this monochrome blue white colour...
and then, the "cree" or "luxeon" LEDS are far superior to plain old 5mm. depends...do you want to see, or simply be seen? you need a fair few of those pre wired/resistored ones to make a decent spotlight.