The crowds can be insane. On our first two trips, the parks were a complete delight. But, we first visited during the GFC and our second visit was shortly after the earthquake and tsunami. On our third and most recent visit, the parks were crazily busy. I think in part it was due to our first day coinciding with the commencement of the "Halloween" events, but also simply because the parks are typically busy. It was also raining, which probably deterred some visitors but which also meant that a lot of rides were closed and a lot of entertainment (e.g. parades and fireworks) were cancelled. That meant there was more demand for the rides which were open.
It's important to prioritise the rides, parades or experiences which you really want to do. And even though it is crowded, the cast members and visitors are all so polite and considerate that it's still a joy to experience just wandering around the parks and taking in all of the scenery, especially at DisneySea. We also found that taking a break from the crowds for more relaxed meals helped.