The problem is knowing which bus takes you to where.
Getting to cancun downtown is easy and is our base to get anywhere else, you have to take a bus named ADO (in spanish Ah Deh Oh :P ) this bus will get you to cancun downtown for around 5 dollars, it has air conditioner and sometimes they play a movie.
If you are staying in a hotel in cancun downtown you can probably be able to walk to it from the ADO station, if not, feel free to take a taxy cab there, in cancun downtown they wont charge you more than 3 dollars if you are going inside the city. if you are going to the hotel zone DONT take a taxy they'll bleed you.
To get to the hotel zone you can take a bus outside the ADO station, buses to hotel zone have the words "Hoteles - Downtown" on it, usually is the buses with the number 1 and 2
You can tell the bus driver to leave you near your hotel, they know every hotel in the hotel zone and will tell you when you arrive.