In September, I responded to an ad from Credo Mobile for a free S4. I switched from Verizon (Moto Droid Razr Maxx) to the S4. I rooted it and upgraded to 4.3 at the earliest opportunity.
Problems:
1. After connecting the phone via Bluetooth to my two vehicles, incoming calls resulted in the caller not being able to hear me. With one car, I followed the correct instructions and re-connected the phone. On the first incoming call, the problem appeared to be fixed. However, on the next incoming call, the problem was still there. I found that turning off the car, shutting down the phone, and reconnecting the BT would fix the problem, at least on the first incoming call. However, on subsequent calls, the problem returned. Conclusion: BT connection was unreliable. Response from tech support: No one else was having the problem; unless I could recreate it so tech support could diagnose it, there was nothing they could do.
2. When the BT was working, the volume control function on the car was inoperable. I couldn't change the volume level using the head unit's volume controls (steering wheel or head unit knob).
3. When on wifi, I typically had no wireless coverage. I rarely had 4G coverage, even though I live in the DC Metro area. My provider was Sprint.
Bottom line - I returned the phones (I had two!) and returned to VZW and our Droids. We had none of the problems we had experienced with the S4s. I am resisting the ads by VZW and others to replace my Droids with VZW-provided S4s. SamsClub is offering them for $0.96 cents with a new 2-year contract.