I've visited several major aquariums across the country, and this ranks among one of the better. It's not huge, and there aren't any over-the-top type displays like the whale sharks in Atlanta or the shark corridor in Baltimore. However, it really showcased local and exotic specimens in a tasteful and engaging manner.
We visited with our nieces who are ~2 and 3 years old, and it was the perfect size for them to walk through and have a great time without taking too much time. It's well thought out from a child's perspective, with easy viewing from their lower point of reference in most of the displays. There are also touch-tanks and other hands-on displays they were really drawn to.
We will definitely return and make this part of our next Seattle visit.