we went on our first haven holiday last summer and we're just about to book our second.
I too had never stayed in a caravan and had pretty low expectations but we opted for haven after seeing the prices at center parcs rocket to obscene levels over the last few years (we have to go in school hols)
We went to Seashore camp near Gt Yarmouth and dd aged 4 loved it and was kept amused very easily. My advice would be to go for a top end caravan if you want to spend any time in it as the cheaper ones are not particularly comfortable. We are booking a privately owned one this time and saving about £400 off the brochure price (obviously ahve to still pay for entertainments/pool passes but it is much cheaper)
The pool was fine when we went in august last year, not massively busy but they are much more basic than center parcs. There was also a beach right over the road from the camp which was nice.
The only thing I would say is beware of all the money spinners at the park, bike/kart hire, "courtesy" bus into nearest town, amusements, laser swords flashing sunglasses, bingo, prize draws etc etc etc sold EVERY night at the children's show etc etc.
The entertainments were good for dd, not brilliant for adults, although there was a circus one night and a michael jackson impersonator who was quite good. DD liked the kids clubs too, they did cooking, crafts etc all completely free.