We just got back yesterday from a 7 day stay, 3 without a cart - 4 with.
I have to say that I definately enjoyed the cart and it certainly didn't sit around our site, not with my teenagers. They loved driving it around the campground, going around the loops, taking it to the pool or fireworks. We didn't have to wait for buses either, which was a plus.
Coming from a state that's close to Canada, you can imagine how the Florida heat affected us. OMG I thought I was melting. The cart was an answer to our prayers. When you add the price of a cart onto the cost of camping, it comes out to be about the same as a budget resort hotel I suppose....if price is a concern. But really, when you're there look around at some of the RVs parked there, some are probably worth more than our homes. Somehow I don't think cost is an issue with many who choose FW rather it's the campground itself. Without a doubt, the most beautiful campground I've ever stayed at.
If you can afford it .......get a cart. You can boat to MK and take a bus to MGM from the Settlement bus stop, but you have to catch a bus at the Depot bus stop for Epcot (etc.) and that's quite a walk --- in the HOT florida sun.