The bus system runs 24/7 and is VERY safe! I have been extremely drunk and took the buses back alone and had NO problems. After all, you wont be the only tourist on the bus, I can tell you that!
Just like any other busy place... If you pay attention to your surroundings and don't leave your drinks unattended you will have the time of your life. bad things happen everywhere, but rarely to people vacationing in Cancun.
You will get a "Safe" ride in a Taxi, just expect to pay out the ass. Both my brother in laws are "Taxistas" and even though they are great guys they charge the tourists out the ass. Excpect to pay anywhere from $5usd to $10usd for a trip in the Hotel Zone.; But you will be fine. ALL Taxis belong to a union called "Syndicato de Taxistas" and they take pretty much an oath to keep their clients safe. You will be fine. I have been here fro 6 years, taken buses, taxis and have a car (Which is the most dangerous) and NEVER had a problem. (BTW I am blonde haired and blue eyed so I certaintly can't pass for a local)
I, too, have always felt safe. But, it is good to hear these stories in order to remind us to always be careful and aware. Thanks!