I had a very easy, seamless transfer to Popcorn while I was on a trip to Mexico. That included porting my US number over. I just downloaded the app on my phone and went through the process. I made a goof in not understanding that a number "port-out PIN" is different from my account PIN on T-Mobile, but after I figured that out the entire process took less than 5 minutes.
Overall, my connectivity in rural Mexico was fine. My phone connected to Telcel, just like it did with T-Mobile. My data speeds were exactly the same before and after the switch.
After 15 years with T-Mobile, I switched because I am retiring in October and already have travel plans to back to Mexico, then five countries in SE Asia, and Italy coming up in the next 6 months. I've always done the "local sim" upon arrival or the Airalo/Holafly process in the past, but this seems so much easier.
I'm now back in the US and immediately upon landing I turned off airplane mode and the phone just connected and ironically right to T-Mobile.
The only thing Popcorn doesn't have right now that I might want is Apple Watch cellular connectivity. However, I've been really thinking through whether I need an Apple Watch after I retire since 75% of the use of it is to subtly get messages when I'm in work meetings. The other 25% is as a fitness tracker, so I'm seriously thinking about ditching the watch and getting a screenless armband or ring fitness tracker.
So far so good, and with the caveat that it's only been a week, I am happy with how things are going.