Playa del Carmen and Cozumel are nothing like Cancun. Cancun is useful for having an international airport to fly into at significantly reduced fares. The thing to do at that point is transfer directly to Playa del Carmen and then jump on the ferry over to Cozumel.
We have done this twice in the last couple of months and are headed back with another group in February. We have wandered the streets of Cozumel and visited many neighborhood restaurants. It's an absolutely delightful place to be, as is the tourist district of Playa del Carmen. Cancun is a different story...