I've fished the flats on the north end of Cozumel a few times. It's an easy ferry ride across from Cancun. Get the ferry in Playa Del Carmin. Nacho Euan is a very good guide there on Cozumel.
As far as Cancun itself, I have not fished it, but have seen several mixed reports. Yes, take your own gear.
Your best bet if not Cozumel, IMO, would be to go south to Tulum and beyond to Punta Allen. Several really top notch operations working out of there.
On further south is Xcalak and the Bay of Chetumal where some really great flats fishing awaits. I've fished it several times and its a great place to go....but you need to take a few days to do it right.