I take the Seneye PUR values at their word (maybe mistakenly), since I have no way to calibrate.
Same here. My feeling is that the results are good enough for our purposes. I actually bought it for setting up my indoor hydroponics where I needed to balance light intensity, evenness of illumination, and also to balance the spectrum with a combination of "full spectrum" white LEDS and red & blue strips. It worked really well for this. Seneye tech support were sure the readings should be equally accurate in and out of water.
If you need to accurately measure light output for reviewing lamps or scientific studies then get something else. But for home / hobby use, the Seneye is (I think) more than good enough, especially given the cost. If money were no object, I would of course have bought the latest Apogee though!