Following on from Chana2's review we also spent a whole day here and thought it was excellent value.
Couple of tips:
You can buy your tickets before you go from a 7Eleven which will save you a bit of time at entry;
get the MTR subway to Admiralty and the 629 bus there and back which is really easy;
most of the food places stop cooking at about 6, so eat early or eat left overs; and
make sure you stay for the 7pm light and fireworks show.
We got there at 10am and rode rides and did activities all day. The rides were great, the Jellyfish display is cool and make sure you see a dolphin show.
The whole day was topped off by the coolest Panda ever!
Probably one of the best days we had in Hong Kong and at HK$280 (AU$35) a person is VERY good value.