Just to add (and re-iterate) a few points on renting a car versus Disney Transport...
* Keep in mind, if you drive to the MK... you have to wait in line at the Toll Plaza (Parking is free), park your car, hop on the Tram that takes you to the TTC. Once at the TTC, you still need to hop on the Monorail or the Ferryboat to get to the MK. (If it's early in the morning, TTC is held back till 8:45)
* At the end of the night... yes, the lines for the busses can look intimidating. Keep in mind, the busses come quickly, and sometimes will even come 2 at a time for the big resorts. The line looks long, but it moves.
* If you drive... you still have to wait in the lines for the Monorail or Ferryboat. They will be long, but they also move just like the bus lines. With both the Monorail and Ferryboat, you will still probably have to stand. Both have very limited seating, and rely primarily on standees. Also, once you get to the TTC, you still have to wait in line for the Tram. (Which means folding of strollers)
Ferryboat 15% seated
Monorail 35% seated
Bus 60-75% seated
A lot of people say that they like having a car because they like being able to come and go as they please, without having to rely on bus schedules. However, generally speaking, the bus is usually your fastest option. For example, we'll take a popular resort, Port Orleans Riverside.
Busses run to PO:R from the MK approx every 10-15 minutes. We'll assume you wait 10 minutes for the bus. It typically takes a Disney bus 13 mins to get to the MK, but we'll say 15 mins. So, total time to the front gates of the Magic Kingdom Theme Park: 25 minutes. That includes waiting for the bus.
For a car... it will take 20 minutes to get to the MK Parking lot. 10 mins to get to the TTC, and 10-25 minutes to get to the front gates of the MK, depending on Ferryboat and Monorail schedules. Thats 40-60 minutes.
But of course... like others said, having a car allows for the flexibility of going off property to other Orlando area attractions.
One thing that I would like to mention... if you do choose to rent a car, and drive on Disney property... PLEASE read this...
BEFORE driving on Disney Property... Get a road map of Disney. Figure out where you are going, and plan your route IN ADVANCE. Familliarize yourself with your route. Once you are on the road, DO NOT distract yourself by unfolding that map and trying to drive and glance at it at the same time. Have someone else do that for you. If you must look at your map again, Please pull to the side of the road, or into a safe area.
If you happen to miss your turn, do not stop and try to back up. Keep going, and look for a LEGAL and SAFE way to turn around.
Make sure to follow all speed limits (Meaning, do not speed, and do not go too slow... this can be dangerous), and please use proper directional signals. (No... the middle finger is not a directional signal... lol)
A word of caution... Hotel Plaza Blvd and Buena Vista Drive, near the Downtown Disney Marketplace area is under HEAVY construction. PLEASE... avoid this area if possible. Park at the West Side, and walk over to the Marketplace if you need to. The construction area is confusing enough for veteran WDW drivers.
Another note... Please drive with courtesy. If a car (or especially a bus) is trying to merge into your lane... please let them. Be careful of Taxi's... they can be dangerous, and will cut you off. Also, be careful of Busses... they can't always see you.
And finally... Please... Buckle up... and of course,
DRIVE SAFELY!