I bought one of
this type of light (claimed to be 1800LM but about the same as my 800LM flashlight, both could be much less
). Not as impressed as I thought I would be.
800lm is about typical for those lights at full brightness. If you look up most reviews where they measure they usually come up in that range. 800lm is still very bright for a $20 light. Beam pattern is an issue with all bike lights, especially the cheaper ones. A flashlight tends to have too much of a center hot-spot, while these tend to throw light everywhere. Ideally, you want the goldilocks beam of not too much spread, but still enough to illuminate peripheral areas.