I've found the reception sub-par. The S6 has an issue handing off between towers when on a phone call if they are not all LTE. I had this issue on my iPhone and T-Mobile told me to turn off VoLTE and it was then perfect. We live in an area of mixed LTE/4G with a sprinkle of Edge mixed in occasionally (believe it or not) but the iPhone handled it just fine as did my Lumia 1520 and Blackberry Z30 before that.
Already have VoLTE turned off on my S6 so thinking there's another issue there. Very frustrating, some days, when I drop multiple calls. Not used to that. Hoping T-Mobile can push through a software update to help the handoffs and/or lower radio signal.