The recent NIH meta-study indicated there may be neurotoxic effects at concentrations within an order of magnitude of the recommended drinking water level (perhaps even at just 2x the recommendation), I wouldn't call that "very low".
The best research I've seen on this, from NZ, suggests the neurocognitive effect in typical fluoridation programs is about as close to zero as you can get.
I admit I think people were stigmatized for raising concerns about it before, and find it sort of weird it was added without more safety data, but by the same token I think the most rigorous evidence suggests its pretty safe at the typical concentrations of most fluoridation programs.