If you are in Seattle, you have to at least drive or walk by the building. It is unique architecturally and, well, you be the judge of its aesthetic appeal... I personally did not find the EMP worth the $20 it cost to get in, but my friend loved it and immediately signed up for a membership! I love music, but this was not the outlet that interested me.
They have temporary exhibits that are very interesting, such a photography exhibit of music artists. I saw one that was of the Beatles. It was very interesting to see previously unreleased photos, but $20 is a steep price to pay to see some photos.
Other exhibits are very interactive, where you get to actually play instruments in the music lab and create a little video or take lessons and write songs. Another exhibit was an amusement-style ride that seemed something similar to Epcot Center, where you are in a seat which moves and sways to a video that gives you the feeling you are actually in the movie. There is now a sci-fi museum attached to the EMP, which I have yet to see.
If you would just like to see the inside of the building for its architecture, you can do so by going to the Turn Table restaurant or the Liquid lounge bar for no admission price and order a drink. The Sky Church (auditorium) has great concerts and shows. Check their website for the line-up.