So renting a car in Cancun is only $1-$2/day with the exchange rate. Can anyone think of a reason not to rent a car for a week as opposed to paying for transfers? The last time I was there, which was ~6 years ago, we did get stopped several times by military that searched our car - leaving $20 in the cup holder seemed to help us from ever being held up (don't ever 'offer' money though!), not that there would be any real reason. On 2 occasions we were told they're only there to keep drugs/guns from "other" cartels out of the tourist areas - what has your experience been?
I don't even like thinking about, or operating a car when at home, let alone when I am on vacation. No matter what the low cost is, having to participate in any type of bribe without offering to me just seems like way more trouble then it is worth. Good luck though, and have a fun trip!
I personally wouldn't ever consider it even if it was free. Sounds like 10,000 x's more stress than it's worth. Also from what I've seen of the driving down there, I'm just happy to get to the resort alive. An accident or fender bender would be a quick way to ruin your trip for no reason. Plus I'm usually drinking on vacation so that doesn't mix either. I've never heard of anyone wanting to rent a car in Cancun before, most likely for good reason, but if you're dead set on it, good luck!
