Hi - Went to St Ives Holiday Park last year - there are definately two sides to it:
The plus side is the location - you can walk through the holiday park to the beach which is beautiful and sandy. It has evening entertainment for the kids, usual stuff like kiddies disco etc. And for those that like that kind of stuff, it had a clubhouse that put on intertainment, bingo etc. It is a very large site and does have some good outside pay areas for kids, particularly down near the beach.
Unfortunately from our perspective it was more bad than good. The facilities were dirty and didn't seem to be cleaned often enough, the pool down near the beach was very cloudy and there were no changing rooms - which took us quite by surprise at the time as there is nothing to indicate that you can't get changed anywhere.
The clubhouse was very smokey and very busy. There was a lot of noise until late in the evening and all in all, not what we would consider a quality site.
There is a caravan club site just down the road called Godrevy which is excellent if you want something cleaner and quieter, but it doesn't have a pool or clubhouse - personally something that we can live without.
On the plus side, if you are fortunate with the weather then I guess you will spend most of your time down on the beach. The beach is excellent and from memory goes from St Ives right through to Godrevy lighthouse.