I always rent a car now.
First, in almost all cases I have been able to rent a car, even for an entire week, for little more, and often less than Mears charges.
Second, with the exception of the Magic Kingdom, I have found that it is almost always faster to drive from my hotel, even a Disney hotel, to any Disney park.
Having a car gives you a great deal of flexibility. Not to mention, you never have to wait for the bus or shuttle, deal with others screaming children at the end of the day, etc.
If you think you might want to go anywhere off of Disney property, even to a restaurant, pharmacy, or grocery store; a car is a must.
To avoid the lines or delay in picking up a car, I:
- Always rent from a company that is at the airport. Offsite rentals can add a great deal of time.
- Avoid Alamo. It always seems to have the longest lines. Other agencies I have often been able to get my rental agreement, even before my luggage is ready to claim.