I used the system on the B744F for 8 years. It is not particularly accurate and can suffer errors from ground slope etc. However the great thing about it is that it shows in small font on the FMS what it thinks your TOW and ZFW weight is so that when you manually enter the final figures you can instantly see if there is a gross error. On all except one a/c it was a dual system.
We had to investigate if the figure was more than +/- 5,000Kg different from the loadsheet so we would certainly have picked up a 100t error. It was also particularly useful to see how loading was progressing and estimating when the aircraft would be ready for departure. It was sometimes necessary to move the a/c to check it i.e. after push back.