We passed through Everett on our last vacation and signed up for the tour on the spur of the moment. I've done most of the tourist things in the Seattle area, and I'd recommend this as much as any of them.
If you fly at all this will interest you. If you are interested in planes its the coolest. Its amazing to see so many brand new planes in one place, and to see how they are assembled. There is also some good history of Boeing and its related companies.
Really an amazing place. There is literally nothing else like this factory any place else on earth. Well worth the short drive up from Seattle.