Decided to fork out the quite pricey admission fee to visit during the Mosaicultures International and the Garden of Lights. Both of these events are well worth seeing as they are very beautiful displays. The Mosaicultures which ends by the 29th of Sept is made up of about 40 sculptures made from plants basically. Some of them are huge and they are all amazing to view. The Bird Tree took them like 6 years to make, thats how much effort goes in to these. The Garden of Lights is an annual nightly event through the Fall where the Chinese Garden is filled with Lanterns of all different things from animals to warriors and the Japanese Gardens is also lit up. This is also very nice to see. Admission during this time allows you two visits if you were not visiting at a time to see them both or I just passed the ticket on to someone else.
Admission also includes entry to the Insectarium which is interesting but didn't take me more than 30mins to explore. During the year when these events are not on, admission is cheaper but still a little expensive so you would really have to appreciate the gardens to pay the fee. The Gardens are quite big and you could spend a few hrs just wandering the paths all the way to the other end where the Tree House and arboretum is and the green houses are quite good.
Overall, if you're going during these events be prepared for lots of people but otherwise pick a nice day and enjoy the gardens.