I've been there. Was lucky enough to sign up for a VIP passes.
We went on the 2nd weekend and it was crazy. Make sure you can buy tickets before you show up. If you get the VIP passes they have a signup deal where you can book times and are guaranteed to get in. We didn't do this, but got the name of the group booking lady to reference to let us in.
The best time to go would be on a week day morning when all the kids are in school, etc.
Being the biggest aquarium in the world I was a little disappointed figuring it would be more walking areas and viewing tanks, etc. But you have to keep in mind it's the biggest aquarium in the world and they designed it in such a way where there is less walking around then one would expect.
Definitely worth at least a one time ticket and a follow-up to the reef in a year or so when it starts to mature.
The whale sharks are absolutely amazing and the school of stingrays swimming over head in the tunnel is absolutely surreal with the light shining down through!
Rob