We always get pesos from the atm, and we use them anytime we leave the resort. It is easier than worrying about conversions. We have used the atm attached to the very small store next to TTR (practically in the parking lot), as well as machines that are part of a large bank (can't remember which bank) in both Cancun and on Isla Mujeres. We have never been charged more than a few dollars for a service fee.
As a rule bring I'd bring a lot of 20 and 50 peso notes for tips. as has been said here before, it's easier for the staff to spend them and they don't take a hit on exchanging it. If you want to break a US 10 or 20 or don't go to the front desk, ask someone in housekeeping, they end up with a stack of them from room tips. Or ask one of the bartenders to make change from the Tip Glass!
