I would like to summarize what others have said on this topic. My family had a fairly positive experience using the certificate my son won at Nationals this year. We had no issues working with the AMEX travel agent and feel they provide a good customer experience.
But, the main issue is that by using AMEX travel, there are significant additional costs involved to use the travel certificate(s).
1. The price of hotels, flights, cars, etc offered by the AMEX travel agency are typically more than what can be found online, and they will only price match the airlines sites. They do not price match against Travelocity, Orbitz, etc.
2. If you do not have enough money on the certificate to make a purchase via the AMEX travel agency, they charge a 1.5% fee to use a credit card to cover the rest of the purchase.
3. The certificates cannot be used directly online. You have to work with an AMEX travel agent, and the travel agent may have to verify with Pokemon if the travel qualifies to use the certificate. I.e. the travel has to be Pokemon related.
4. There is a $30 booking fee per flight. If a family of four uses a travel certificate, they will be charged $120 in fees to use it for four flights.