Disney transortation is comprehensive and will get you anywhere you need to go on-site.
But I always rent a car, even when I'm staying on-site.
Advantages of having your own car and self-driving are:
*Never wait for a bus
*Always get a seat
*Never wait in line in a bus queue
*Saves TREMENDOUS time, every day!
*Easier to fold and store strollers (n/a for me but a major issue for families)
*Ability to go off-site for shopping (outlets or snack supplies), dining, and other venues (Universal, SeaWorld, dinner theaters, etc)
MK is the one trip where self-driving is not faster than taking WDW transportation, because the MK lot is on one side of the Seven Seas Lagoon, and the MK gates are on the other side - and the buses drop you on the gate side, not the lot side.
However, when you park at MK, you either have to take the ferryboat or monorail across the Lagoon to reach the park - and I LOVE taking the ferryboat and monorail! As far as I'm concerned, my Disney Day starts the moment I step onto the ferryboat pier in the morning, and it doesn't end until I step off the monorail in the evening.