I had am HTC shift before this phone and it had its issues for sure, however I could make phone calls at the mall and in other stores. My gs3 is horrible, I don't have enough service to call, text or any data when in stores I used all of them in with my HTC.
I am in the Metro DC area, and I can report that over the last SIX months service has ranged from non-existent to spotty to "wow, I didn't know they had 4G here". I've read reports that Sprint was doing tower upgrades in this area, but whenever I feel like I'm getting no signal I reboot/restart my S3 (stock 4.1.2 MD4, unrooted) and my signal mysteriously gains about 2 bars (enough to make a call etc). If it's really bad, I update my PRL (Settings > System Update) but I don't know if it has any long-term effects with the signal.
At my home in suburban MD, I've been lucky to get 2-3 bars of 3G (no 4G). Our home borders a park forest and we have a LOT of trees in the yard, if that still makes a difference with new technologies (tree buds used to be to blame years ago). When I need to make a call, I try to be near a window and turn off WiFi.
I would like to say that my Epic 4G had a better signal, but if system upgrades are the cause of signal drops then you can't really compare devices.
cheers/dom