That's a tough question. We always rent because we go all over the area diving. But renting a car in that area can be stressful. Always take the extra insurance as well as booking with a credit card that has car insurance. If you get into an accident with a local, I'm sure the federales will take the word of the local over you. There's rumours of people getting jailed until their credit card company pays off the damages. That's why we pay extra for the local insurance. Adds an extra $20-$30/day to costs though.
We've never used the collectivos (local buses along the highway), but have heard that they're convenient. There's stops right outside Puerto Aventuras, but it might be a bit of a walk from wherever your condo is. But you can travel up and down the highway for cheap. Not sure what a taxi costs to Playa from PA.
It's nice to have a car to go get groceries or to run into Playa or Tulum. A few warnings though:
1) Always slow down for the speed bumps!. If you see a sign that says "tope" slow down. They definitely go a little overboard with the size of their topes.
2) Don't get distracted at the gas pump. Watch them zero the pump and watch it as they fill your tank. Ignore the guy asking to check your oil or fill your tires until after they've filled your tank.
3) Don't give taxi drivers the finger. They have a very strong taxi union that even the police respect. They'll literally box a car in and make them pull over.
4) Remember you're in a foreign country. Don't leave anything valuable in the car...ever!
We've never been harassed by the police unless we did something stupid like speed. But we were travelling with an ex cop from Canada that would flash his badge and trade a few patches. So we've never had issues.
In the long run, you'll have to balance the costs of renting a car against taking a taxi from the airport, to get groceries, perhaps to get to your class, to go out to eat, to go exploring versus the convenience of having someone drive you around and never getting lost, not worrying about having a car broken into, getting in an accident and finding parking.