I purchased one light, and while the light is bright for the price, I could overdrive the light going downhill. I'm on a budget, so instead of purchasing a [$] light I purchased a second one, and the two work great. I've ridden with them for 4 years and have been very satisfied.
Having two lights mean that you are never out of light in case the batteries discharges - there is always one light working.
There are two drawbacks a) the center of gravity for the light is forward of the handlebars, so if you are on a very rough road, you run the risk of the light tipping forward on the handlebars, and b) if you don't clip it on the mount correctly, it will pop off and roll along the ground (pilot error).
The brightness is probably the best in this price range - a good value!
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