Traveling with an Amex - will I be able to use it widely in Micronesiaor should I carry a backup Visa/Mastercard?
Yes, you can use your Amex, but it’s not as widely accepted as Visa or Mastercard. I was there last spring, and found that my Amex worked at a few of the larger hotels and dive shops, but beyond that, it was a no-go. The best strategy is to have both a Visa or Mastercard for your primary card, and plenty of cash. You can get US dollars from the Bank of Guam ATMs with your Visa, but you won't be able to with your Amex.
It's a mixed bag. I was just there last month and used my Amex at some of the bigger places in Kolonia, like the Joy Hotel. However, most of the time, I relied on my Chase Visa card. The Amex was more of a last resort, and I never saw an ATM that accepted it. My advice is to assume your Amex won't work everywhere and carry a backup Visa/Mastercard for peace of mind and cash for the rest.
Absolutely bring a backup card. While the US dollar is the currency, and some places do accept Amex, it’s not as widespread as you might think. Many small businesses, guesthouses on the outer islands, and local restaurants only deal in cash. Even some of the bigger businesses might not be set up for it. I recommend bringing a Visa or Mastercard for flexibility and also having a good amount of cash on hand for tips and smaller purchases.
You can use your Amex in Micronesia, but it's best to have a backup. Major tourist-oriented spots and some hotels like the Village Hotel in Pohnpei will likely accept it, but don't count on it for smaller local businesses or remote islands. You'll find that the Bank of Guam is the most common bank, and while they handle Amex merchant services, their ATMs are primarily for Visa and Mastercard. Having a Visa or Mastercard will save you a lot of hassle.