I stayed there last month with a group I was with. The thing I like most about the GWLs I've stayed at (Mason and Sandusky) is that the rooms are quite large and nice. The waterpark itself is fine, though it isn't really that big so it can get quite crowded in there. Apparently there used to be a water coaster there, but it seems it was removed due to maintenance issues and I wasn't really impressed by the slide they replaced it with. (Weirdly enough, it's named River Racer, though it opened before Kings Island's River Racers) The food there is also okay, but it is overpriced. There is MagiQuest there, though I personally didn't do it.
Personally, I do think the GWL in Mason is better than the one in Sandusky since it has a slightly larger water park. The thing with GWL is that they have a lot of locations across the country, but I find their waterparks to just be okay. There are better resorts out there that give GWL a run for their money, like with Kalahari and the Wilderness Resort. However, those you have to go out of your way to travel to, whereas GWL is a lot more accessible, especially since it's right across the street from Kings Island.