I have never had a bad experience at the Carousel. It is my top choice when planning a trip. The only time I stay elsewhere is if they are full.
We've arrived early but have never needed to get to our room. The staff gladly stored our luggage for us (or we left it in the car) and just went to the parks.
Yes, the pool is on the roof. You have to take the elevator to the 5th floor, then walk up the last flight of stairs. Like most of the places right there on Harbor it is a very small pool, but we're there for the parks, not the pools.
In my opinion the Carousel has the best continental breakfast of the ones along that strip. You get muffins, danish, toast, jelly, oatmeal, fruit (usually apples and either bananas or oranges), juice and coffee. Some of the other places only have danish and coffee or juice, if they even have anything at all.
I've stayed in a 2 bath suite, which is basically a regular hotel room with 2 double beds and a bathroom, then an additional queen bed and full bath closed off by a locking door. I haven't stayed in their view suite, but I've heard they are really nice and people have gotten great rates for them lately ($159/night). These rooms are at the end of the hotel, facing the parks and you overlook the main entrance to DL and DCA.