I've been to the Museum of Flight every time my in-laws come into town and I'm SO over it. Unless you're really into planes or have A LOT of time to kill doing nothing, it isn't really worth it.
Everyone has described it correctly. Lots of different planes to see. Some hanging from the ceiling, others on the ground.
The last time I checked, the first Thursday of each month was free to the public. That's the BEST time to check it out.