I had the $20 magicshine clone that could last about 3 45-minute rides (on low constant setting) before needing recharging, but one day it suddenly died during my ride without warning---without going dimmer, just from bright to completely off.
Plenty of the cheap lights are OK but they usually rip you off on the batteries, often getting a battery from a reputable source will double your run time. I use Hunk Lee on ebay, the genuine magicshine stuff is probably a good bet too.