We used to rely on Traveler's Cheques quite heavily. But, as Craig said, redeeming them has become more of a hassle. AMEX wants to be called for a verification code to cash their checks. Many Casas de Cambio ask for a Passport - which we're not in the habit of carrying on a day-to-day basis down there.
The prevalence of ATMs on the Riviera Maya makes Cash Cards a much more attractive option. We opened an account with Bank of America specifically for the Cash Cards that can be used at their Global Alliance ATMs.
Dave