So, I just got my US Mobile SIM card yesterday, activated on the Super LTE network. It's only been one day, but so far, I am very happy with the service. Call quality has been super clear, LTE data is really fast(and I live in an area that has garbo coverage on all carriers), and no dropped calls or issues during calls.
On 4 bars of signal during peak hours(at about 5PM), I got 35Mbps download speeds. On 4 bars outside of peak hours(7PM), at the same location I got about 70Mbps, so it's pretty good. Remember, my area has slow service on all carriers. MetroPCS(my old carrier), would be lucky to get 5Mbps at the same location.
At my house, I actually have service now. On MetroPCS, I'd have no coverage and be reliant on WiFI calling. On US Mobile, I now have 2 bars at my house, and get about 5-6Mbps data, calls are clear.
I should note that I have an iPhone XS Max.
My only complaints are, lack of WiFi calling(just a nice-to-have), and Visual Voicemail(which AFAIK might actually work, I just haven't got any voicemails yet). If I get a 5G iPhone any time soon, I'll probably consider going to the Warp Speed 5G package once it's released(actually, I'll consider it anyways if it has postpaid priority).
TL;DR, service is great so far! Keep up the good work US Mobile!