We're registered & roaming free on T-Mobile's Global data both in Taiwan & South Korea earlier this month, all around the region in both places for 7 days each - Facebook uploads certainly worked great, fast enough to upload quite a # of pictures while on the move (inside cars/taxis & buses) & seldom lost our signal. Maps were very much functional and of course, more than adequate to check & read emails/attachments, etc. - and made WiFi calling on iPhones' 6 and 5s. Also did Facetime video & Hangouts calling back to the states, and between TW & SK too (Google billed it at USD 0.04/minute)

SKTel also has a "5G" network, possibly compatible ?? with T-Mobile but I couldn't connect to it to see, just out of curiosity, on my i5s as that's a prepaid backup line with no access to roaming data (ditto, same for the iPad Mini with FDFL on home network only) - send/receive text on T-Mo prepaid would be 50 cents each on SKTel - voice call would be billed at $2.69/min ... that's "expensive" but not for emergencies, if I can't be reached on other numbers for whatever reasons.