For the Pi camera with 14cm fiducials mounted on the ceiling with a ground based robot, in a well lit room we get position data that is good down to a couple cm. The biggest problem is noise in the pose estimates of single fiducials, as a couple pixels of noise can change the angular estimate wildly.
We have a method in the works that instead of doing individual marker pose estimates, does a more sophisticated approach using a single pose estimate on all the detected marker vertices. We want to have the data set and testing to measure if it is actually an improvement before merging though.
Rohan