Back from our two month trip to Europe, UK and NYC the app worked well in mapping without using internet. I used it a few times to check the route to somewhere. While driving in the UK I used my Garmin NUVI with a European chip that I got cheap on eBay. Walking around in foreign countries was easier to use Google maps since they have a walking mode. The MAPS.ME app only supports vehicle routing.
It was cool to watch ourselves running down the RR track at 180 mph on EuroStar. For what we used it for, it worked great. Now I need to see how to delete the maps that I don't need.
For internet and telephone I used T-Mobile. Unlimited slow speed (128 kb) data. Worked ok for Google maps, email and light browsing. In Britain, Wales and Scotland we used GiffGaff sim. 15 pounds ($23) for 500 minutes phone, 3GB data and unlimited texts for a month. Too bad we don't have an option that cheap in the US.
That's my report. I'll continue to use Garmin in the coach and toad.