My summer trip to Seattle just happened to be the same week that the new Science Fiction Museum opened at the Experience Music Project. A motion-control ride about "funk music" has been removed from the EMP building, and the space turned in the new SF exhibits.
My dad and I went the Monday after it debuted, right when the museum opened. No crowds whatsoever. I recommend going early in the AM to anyone, as that seems to be a great way to have the time and space to look at whatever you want.
I found it to be an eclectic and small collection. Obviously this thing will grow over time, but for now, it took us only about an hour to look at everything. What is presented is a good overview for folks not familiar with the topic, and whereas fans will see some faves and some rarities, presently there isn't much you can really sink your teeth into. In my opinion, if they had more space, they could expand all the exhibits and cover more material. I liked what was there, but really wanted more of everything.
They have a web site where you can get an idea of what is there.
http://www.sfhomeworld.org