You may check with your hotel as well, they may have some sort of transportation. However, we just did a cruise in October and we used the transfers (we spent 2 days at one of their resorts before going to the ship, then went straight to the airport). We went with another family and there were 7 in our party, it was cheaper to do the transfers than rent a car (especially just to have it sit all those days). The buses they use are really nice, they show a little video, and it was totally hassle free. All things being equal, I'd do the transfers again.
And by the way, it was the BEST vacation EVER. We all loved it, bet you'll have a blast!! Let me know if you have any questions and I'll try to answer them. Really, I'm telling everyone I know to go on a Disney cruise, we can't wait to go back!