When we stay at a Disney resort, we almost never rent a car. I find dealing with picking it up, driving myself to the resort, and back to the airport to return it a hassle. It's not a huge one, but I would rather use a towncar service or magical express.
Mind you, I hate waiting for the bus with ME. Hate it. First time we took it, we waited so LONG - it was well over two hours until we got to the BWI that trip (and we took a taxi back that trip, not wanting to deal with it). Now I use it and find something to kill time in line (we've always waited at least 45 minutes between the line and waiting on the bus).
Off site, of course we have a rental car (and that's almost always been at a condo, so we're always stopping at the store for various things like food and drinks). Then, of course, we drive to the parks.
One thing I like about a Disney vacation is not driving. And, last trip we stayed at Pop Century - we never waited for very long for a bus! It was nice that the bus isn't shared, so it was very convenient (and DD uses a wheelchair although she's ambulatory - so they let us on first so she could sit and I'd fold it up out of the way - so convenient).
I'd personally save my money and use ME if staying at a Disney resort. I do like the quick food/drink grocery stop that I could do with a rental car or towncar, but would rather not drive. We ordered from Garden Grocer for our upcoming trip (AKV Kidani).