We live so close to the Dells. My 10 year olds school recognition trip was to Chula Vista this spring. He loved it as much as the others. I haven't stayed there since the major remodel- think the last time I stayed there it was the late 1980's. The current ads they are running make it seem like it is very nice.
The older one went to Mt. Olympus for the day for his 8th grade trip 2 years ago. They like the indoor water park and the theme park. We have also spent the night at the Wilderness, the Kalahari, the Great Wolf. We have decided that if our transportation costs weren't so cheap (a 30 minute drive) it would be cheaper to go to Disney World on a per day basis (and it is).
Don't forget there is much to do besides water parks. The Dells boat tours (I like the upper Dells much better than the lower Dells), the "Ducks", golfing, Mirror Lake State Park. Tommy Bartlett's ski show, I have never paid for it...I always go and eat at the Rivina Inn- the deck gives a perfect view of the show.