We went for a week last September. To be totally honest, although I loved it because it was Orlando and still enjoyed the parks and everything, we did feel more under pressure to fit everything in. In order to do everything we wanted, we had planned no rest days but, by Saturday (we arrived on Tuesday) we had to take a break as we were both starting to get really stressed and tired.
As it was only my second time visiting, I've no idea if we enjoyed it less because it was only a week or whether it didn't have that special 'first-time' feel. However, we both agreed that we wouldn't go just for a week again, unless we had been loads of times and were only going to limit ourselves to a couple of parks.
Just wanted to add that we didn't do a lot of the stuff we wanted to, after all that, because we felt so under pressure and needed a break so we had to forego some of our plans. Don't get me wrong, we had a good time but, personally, it's not worth it to us to go for less than a fortnight. In terms of costs, aside from spending money, you can go just as cheap for 2 weeks as you can 1.