While using AMEX travel's website can have its advantages with the double points on flights and in some circumstances hotels, I often find its not worth it, at least for flights.
Why? Because it sucks in terms of selection. Even if you have a simple round-trip flight it often doesn't list all available flight options. Tends to only offer a few. If you have a multi-destination itinerary than you might as well forget it. I personally don't use the other online travel sites either for the same reason. If I find a slightly complicated itinerary on ITA Software that I can't book directly on the airline's site I just do it over the phone with the airline, you can't build them on any of the internet travel sites.