We have stayed at the Great Wolf Lodge in Wisconsin Dells, and loved it.
As far as the spa - the first time we stayed there - in 2 days - I was not able to book an appointment - I think I was going for a manicure or a mini massage. They were just plain booked. The second time I stayed - I booked a manicure for me. It was very nice~I should treat myself more often!

The spa in the Dells is an Aveda spa. When we go back...I'll be back at the spa.
As far as the waterpark - our primary reason for going was for DD to get together with her cousin. First visit (2004), they were 10 and 11, Last visit (2005) they were 11 and 12. Next year, the cousin will be a freshman in High school, and I inquired if both DD and my niece were going to be "too cool", "too grown-up" etc to come back to GWL. They both assured me that they will still be able to find plenty to do.
They loved the Wiley's Woods part of the too - if that is something your GWL has too.
While in the Wisc. Dells area, there are many indoor waterparks (DD loves the Kalahari, we've never been to The Wilderness), we find that GWL offers the best deals for my tiny budget. The 2 times we've been there, our room rates were $119, right before school started, when the bookings for the hotel are way down. We have not had good luck trying to find similar deals at either The Wilderness or the Kalahari.