I have a couple of those and they're excellent as a 'be seen' light, will light up reflective surfaces a couple blocks away, but aren't really bright enough to drive with in complete darkness.

Not sure what you meant by "won't work with 31.8mm bars." The clamp has rubber spacers and will handle that size by removing them. The light isn't fully waterproof, but the venting is on the bottom and they've handled heavy rain fine so far.