Plan ahead for an amazing experience!
This is a great aquarium and you can have a terrific visit with a little bit of planning. Yes, it does get very crowded later in the day. Arrive before opening to get in line and then visit the most popular attractions first (Ocean Voyager and Cold Water Quest). Afterwards, have an early lunch before touring the rest.
Later in the day, if you can afford it, the behind the scenes tour was really interesting and informative. The group was under 15 people and it went to all the uncrowded locations behind the scenes.
We definately plan on returning. The Georgia Aquarium is new, modern and has many things that you won't see anywhere else. They are also in the process of adding on a large dolphin exhibit in 2010 (which is why the penguins are currently off of public viewing except for the tour).
For what you get to see (things that cannot be seen anywhere else) it's not that expensive. And that's also why it gets so crowded.