Are you:
Planning to leave property at any point (for shopping or dining etc.)
Hauling Kid(s), shopping purchases, or lots other stuff around
Can't bear to stand in line to wait for the bus
Can't bear to stand while the bus is on it's way to the resort
Planning to Resort Hop a lot during your stay
Have a strict schedule
Hate sitting/standing closely next to the (insert your favorite adjective strange, smelly, etc...) person beside you
A person who really needs to be in control of where and when they are going
Planning on leaving the park the SAME TIME as everyone else (i.e. rushing out as soon as the fireworks/parade/fantasmic has ended)
You really need a car!
Are you:
Planning to sat on property the whole stay
Carry just the essentials with you. And have everything sent/stored at the resort
Have no problem waiting a few minutes for the next bus.
Have no problem standing in a line to get on the bus and can stand in the aisle while the bus is underway
Usually just go from the resort to a park or parks and back to the resort (i.e. no resort hopping)
Have no problem with the other people on buses
Are easy going. Relax you are on vacation! No worries mon! kind of person
Would rather have someone else worry about speed limits, possible DUI's, finding your car in the lot, finding your way back to the resort after a long day in the park, seat belts, etc.
Have no problem window-shopping, people watching, eating ice cream etc. until the mass of the people are gone.
You are better off with Disney Transportation.
Other than that it is really a matter of personal preference.
Let me explain. I tested the system last year in September (Yes I am a little crazy! We few in that trip and didnt have our own car.) My Boy friend insisted you had to have a car no matter what. I said we only needed one if you leave when everyone else does or plan on leaving property or resort hopping (which we don't do) and there was no way I was paying for a rental when Disney has free transportation. So we bet on it and devoted a day to trying it out. We drove to the each park in the car then went all the way up to the ticket turnstiles then turned around and he headed back to the car and I took the bus back to the resort. All four parks. Then again and again, again.
Conclusion:
Unless you are leaving during the mass exit after the fireworks/parade/fantasmic where having your own car will save you a good 30 mins or more. There is really little difference (less than 10 min difference than using your own car or taking the bus. How lucky you are in getting the bus to show up quickly determined if the car or the bus got back first. Except from Magic Kingdom the Bus actually beat the car back all 4 times but I think that was just because he took the ferry. (It's so sloooow)
Result: He turned the car back in and paid for a nice dinner with the extra money.
Keep in mind: We live in Florida, visit Disney several times a year are annual pass holders and it's not going to ruin our vacation if it is an extra long wait for a bus for some unknown reason. Only carry only each persons cell phone rain ponchos, 1 wallet and a water bottle all of which fits in a small messenger bag which I carry. (Except his cell so we dont lose each other) All purchases get shipped back to the resort. We go to dinner directly from the park if is not the resort we are staying. We have had to take a taxi a few times to avoid the bus shuffle when we have end up back at the resort and were having dinner at another and time was pressing. (Never cost more us than $20 including tip just call desk from your room and tell them you are coming down now and need a taxi, when you get to the lobby its there waiting like magic!) And most important have no kids, that we may have to carry back asleep at the end of the evening. If this does not sound like you then you need to rent a car.
Have a good time.
