But at the price, I think it's worth considering. Spanninga makes a small, lightweight battery taillight designed to mount on a rear fender. NiteRider makes a wide variety of high quality battery powered systems. For off road use, NiteRider lights provide lots of light with NiCad or Nickel Metal Hydride batteries. Off road riding requires much more light than road riding.
Trails can be very difficult to see, and corners can be sharp.
Speeds can vary dramatically, and the light must remain bright at all speeds. I recommend a handlebar mounted light and a helmet mounted light. The best handlebar light would have a wide angle bulb to show you the whole trail, particularly up close. The helmet mounted spot light moves with you to illuminate the trail ahead as the trail curves. You can always see far ahead to avoid surprises.