We had sailed the Classic Ships 5 times and just got off the Fantasy a week ago today.
As far as Navigation, it did take a day to get used to the layout, but it was fairly easy after that. If you ever have an issue, there are lots of signs letting you know where things are.
I was nervous about the size and crowd factor before sailing, but we actually found the ship to be no more crowded than the Classic ships. Even on Sea Days, there were plenty of chairs. We never had an issue finding a chair even at the kids pool area. I like how they use the space on Deck 12. I always felt that the space on Deck 10 on the Classic ships was "wasted". The line for the Aquaduck was always very manageable. There was never a wait for the minigolf. The Aqualab was empty most of the time. Satellite Falls was uncrowded and even the adult pool didn't seem too bad. I looked into Nemo's Reef as we walked by and it too never seemed crowded.
We never had an issue finding a seat in the theater even showing up a five minutes before Aladdin.
The shops weren't crowded except when the theater let out and that is no different from the older ships. Palo was easy to get a reservation for brunch. Remy lunch was very peaceful and uncrowded.
Deck 4 was never crowded although I do wish they would have walkers go the opposite direction as runners Walkers tend to walk together and block the path.
I was purposely looking for crowds on the ship to try to affirm that I liked the Classic ships better and I never found them. It was a bit hectic getting off the ship at
Castaway Cay. There were more people at Cabanas when it opened in the morning, but again the ship easly accomodated them.
My conclusion after last week's cruise is that the new ships are different, but not better or worse than the Classic ships. Yes, there are some things I like better about each of them, but I would happily sail on the Fantasy or one of the classic ships again. We actually booked the Fantasy again for next summer and can't wait! We won't sail on the Dream until it does longer cruises.
ETA: Our room was near mid-ship so we never seemed to have that long of a walk anywhere.