Enjoyable. Just wish it wasn't so crowded.
As a local, I've visited the Seattle Aquarium about 3 or 4 times. It's not special, and the exhibits are certainly not impressive, but it is nonetheless a good experience if you just wish to see and learn about a few of the inhabitants of Puget Sound. If not for the seals and otters, the touch pools, and the (crowded) underwater dome, a trip to a local marine pet store would provide the same satisfying experience and save you $16.00 per person.
You should plan your visit on a weekday, especially if you're a claustrophobic like me. Otherwise you'll find yourself having to squeeze through all the people and their children just to view the fish.