Any American citizen traveling outside the United States is well-advised to carry a passport.
Passports serve as proof of nationality not only at border crossings but to United States consul officers in countries abroad. While entry into some countries such as Canada and the Bahamas is allowed passport-free, most countries require a passport with a legitimate visa stamped in it to enter. That said, I have found the passport agency to be very efficient. They allow for both normal processing and expedited processing. Expedited processing allows for processing of your passport within five business days and is very reliable.