We visited twice on our August trip. For a ton of pictures and lots of little details about the park, check out my trip report (see the link in my signature).
Is it a must-do? That's hard to say, and depends on a lot of factors. We enjoyed it enough to plan for it on our Christmas trip (weather permitting, of course) and have already booked a cabana for Christmas Eve day. But we are waterpark lovers and love the chance to escape our cold Canadian winter for an Aquatica beach day, so maybe our opinion is skewed.
The park is beautiful, the slides and water attractions are awesome, and its a great place to spend a day. Roa's Rapids is by far the best water attraction we've ever enjoyed as a family, and we have been to many waterparks (including the USA's largest, Noah's Ark, in Wisconsin Dells).
The weather could be spectacular and very much waterpark-worthy in early December. Or it could be crap. I would say that's your biggest and most important variable, and unfortunately, its one of the few over which you have zero control. And won't know if its an issue until right before you travel.