Here are some of my thoughts...
I rented a car last time, but I stayed off property (stayed on property in 1999 and found that the transportation at the time was very slow). But, I don't think I will go next time without renting a car (I will be staying onsite next time). I found that I really enjoyed that freedom more than anything else. Yes, paying for parking got expensive after awhile, and paying for gas wasn't always fun, but freedom cannot be better than the feeling of being able to get around anywhere at anytime. If, for instance you feel like going to the grocery store, you can. If you want to leave early from the parks and go shopping, you can. If you decide, or your kids want to, go to Universal for the day, you can. The thing for me was that I ended up getting a car for 11 days for $170 USD and it was cheaper than transfers and public transportation.
JMHO,
Jo