The radios in the S3 are not bad compared to other Samsung devices on Verizon. In my experience, they're slightly weaker than the radios in devices such as the RAZR/MAXX and the Rezound, but they're still pretty good and very usable.
There are many things that impact reception, and geographic location is always one of the major ones. Since there's a lot that goes into a reception that any given person gets, it's tough to say why some some people get good reception and why some people seem to get bad reception.