I've done it three times so far this year. Its not bad. Buy the bus ticket from the ADO guys at the terminal. Do NOT buy the ferry tickets from the ADO guys! Buy the ferry tickets right at the pier from what ever company is the next to leave (there are three now). I do not buy the round trip so you can pick the next most convenient ferry from the island on your way home.
The first two times coming back to the airport I almost was denied Boarding by United as the bus was about 1:45 travel time(I only allotted 1 hr for the bus and 1 hr for the ferry)... I took a cab to the CUN airport last Saturday $69 for the three of us. Worst part was humping the bags up the hill to the van. Once there we made it to the airport in 35 minutes. I'll probably do the taxi/van from now on, unless solo and up really early to catch the O' dark 30 ferry. I can't afford to miss my flight and the bus seems to take too long recently going to CUN. Terminal 3 (most US flights) is the last stop btw. I'll take the ADO bus to Playa every time, unless in a rush. ADO is nice and really cheap. It's really not bad and if the cost savings is more than $200 on a RT ticket and I can get a direct flight out of CUN after 2:00pm I'll do the bag drag everytime. The key is that the flight is direct to/from CUN. No broken planes or weather in Texas or ATL. Just get to the CUN airport grab an adult beverage and start to relax.
Here is the shuttle people I used:
Cancun and Riviera Maya Tours and Transporation by CARM