I was recently in Oahu. I know, it's still America, but wanted to share my experience. Data speeds were excellent! I never found a single dead spot, even up on the mountains. No congestion what so ever, even in the resort areas. Overall, great service in Hawaii and Honolulu area and I was all over the entire island!
As a resident of Hawai’i (Honolulu Metro), I can confirm that coverage is available over the whole island of O’ahu. However, data speeds will vary with very heavy deprioritizing (i.e., dial-up modem speeds) during peak hours.