I have a RCBS 750 and I have found it accurate up until it starts wandering. I have had 3 and they all done it. I have tried letting it warm up for an hour up to what I do know which is not ever turn it off. It seems to wander less since I have it on all the time. I got me the RCBS 502 beam scale and I'm very satisfied. While for some powders I have to trickle 3 kernels to see a change in the 750 it only takes one kernel for me to see the beam scale move. Plus my beam scale don't round numbers up/down, is not affected by lights or power fluctuations. Because of that the 502 is my powder scale of preference but I still use the 750 to verify if I setup the 502 correct and I also prefer the 750 for sorting brass/bullets.
good luck!