We stayed onsite last year. We have been to disney eight times since the birth of DS15. Always offsite until last year when we thought we would immerse ourselves in Disney and the dining plan, transportation, etc.
We got two rooms at POP, which was great. We have been spoiled by off site house/condo rentals, though, and even though we had two rooms with two bathrooms, we were still crowded. Offsite houses offer kitchens, living rooms, many tvs and bathrooms and space to get away from each other! For us, having a little seperation space makes the vacation even more magical. It was very nice to rely on Disney to transport us everywhere, we didnt rent a car, but you do have to wait around for busses....its all give and take. We LOVED the extra magic hours, but the meal plan was not a money saver for us. It was hard for us to eat that much food, though we did like trying out all the places we usually skip over when we have a car to eat off property. Every vacation is magical, don't set yourself up for disappointment. Both onsite and offsite have plusses and minuses, just plan to have a great time whichever you decide!