I just picked up one of these (16w) from Airbomb for $75, and it is VERY light compared to alot of the Nicad setups, and seems more than bright enough, even for the country roads where I live. Although, if you regularly go 40mph downhill at night, then a 25w would be better.
http://www.cygolite.com/light/produc...verNiMH6hr.htm