We've always had a car with us during family trips (solo and groups of adult friends, sometimes not). When we have it, we use it often... visiting other resorts for meals & boat rentals, offsite attractions & restaurants, grocery & medical needs. The minor Disney attractions (water parks, DD, WoS) are all best reached by car, hands-down.
We used to ride Disney transport exclusively to the theme parks. Now, we only bother with Disney transport if we're going between an onsite resort and MK or there's a boat/monorail for transport, since we enjoy those a lot, in themselves. Waiting around so long in standing lines when tired has been a drag.
Forget the Disney buses - they're smelly, often overcrowded, sometimes require multiple stops (either sharing resorts or around a large resort) and take as long as a car in transit. What's worse, sometimes you wait a long while through several different bus arrivals and finally see your bus, only to discover that it doesn't have enough room for everyone waiting... so you then wait some more. This is especially true at park opening/closing times - when you are the most interested in moving quickly. Ugh.
These days, we prefer to use our car, even onsite, thanks.