I purchased an iPhone (cash up front) and set up Xfinity service last January. My monthly bill has averaged about $6.00 per month, based on usage. A couple months, when I was traveling out of town, the bill was $14-$15, but most months, when I'm mostly home (I work from home-and am semi-retired) where my Xfinity WiFi is active, my bills have run for $3.50 to $4.50 each month. The screen (where the "bars" of signal strength is shown) reports if you're on Xfinity WiFi, Verizon LTE or some other variant of service. I have not had any dropped calls and the service has been reliable (as is my cable/internet broadband service). Traveling to some major cities (recently to the east coast) the service was also as good as it is locally. Road trips "to the coast" etc. the service can be spotty, especially in the low-density rural areas and away from major (high-volume) highways. I tend to stay at state park campgrounds ... and frequently experience "no bars" in those locations. But I encounter service sufficiently to keep up with email, etc.
I previously had a prepaid Tracfone (as standby/emergency) service. It required prepayment, and by purchasing a year at a time I got about 1500 minutes and 12 months service for $120 ($10 per month). The last Tracfone I had was a bottom-line Android LG model (that is, online ratings were at the bottom of the list). I could not use the data function because that was an "extra charge". I also had Verizon data only ($20 per month) for my iPad 2 (that's now effectively obsolete). So for data and voice, that was $30 per month for the combined services.
Before that, (roughly 2007-9), I had a Verizon "flip phone" (voice only) and an "air card" for data service for my computer, while we traveled for 2 to 4 months at a time in a large RV. I no longer recall exactly what I paid for that service, but it was close to $100 per month. Verizon service is generally good, but RV travels, showed that there were MANY holes in the service, most notably outside of large metro areas and especially when away from heavily traveled interstate highways. (And Verizon in Texas was often "no bars.")
I am fully satisfied with the Xfinity Mobile service. For me, the price of the service is very reasonable since I don't use a lot of data, especially away from home (where the WiFi makes it "free"). That said, as a Comcast customer (in two different states) I can observe that their customer support, in general, really stinks.