For our Easter (week) cruise, with 3 adults, 4 children, and 1 infant, we thought seriously about Disney Transportation and two rental cars. We stayed 4 nights at WDW before the cruise, so we knew that we needed transportation while on property. At the last minute, I was able to get a 15-PAX van (which is shocking because it was Easter Week). Through my company, the discount made this rental $745 for the week (all taxes included). I prepaid. (Our discount includes loss-damage insurance.)
We did need to fill the gas tank once -- which cost nearly $100 -- but overall, it was the best fit for us. We had a night after the cruise and stayed at the Homewood Suites By Hilton right at the airport (Samoran Blvd). We were able to take the van and drive to the Premium Outlets and do other shopping. Without the van, we would have been at the mercy of whatever shuttle the hotel provided.
In the end, it would have cost us $560 for the Disney transportation to/from the port. If we wanted to go to the outlets or do other things, it would have easily cost $600 to rent two cars for the week.
I didn't want to spend $750 on a rental 15-pax van, but it worked for us. We ended up using the entire last row for luggage. In fact, we had luggage under the last seat, on the last seat, under the middle seat, and crammed against the door (with a stroller as well). Luggage, luggage, luggage.