100 lumens is not very bright. The bean pattern is important. Some flashlights that just added red lenses have a very tight beam size. That's not good for a tail light. Even with good tail lights most need to be aimed straight and horizontal to be useful. They lose a lot of visibility if they are attached to a helmet or clothing and end up facing down a little. Mounted solid to the bike and secure is important. When getting a new light don't go by ad copy to know what it looks like, put it on your bike line it up right, and walk behind your bike in the dark. Crouch down to be about where a driver would be too. Walk to the sides to get a good idea of the spread. Walk far away too.

Edit: join CandlePowerForums and read up. There are a few reviews and comparisons about the clear lens flashlight version.
Maybe the red lens version too.