I consider myself a museum expert, and I adore this museum. Once you get over the expensive admission fee (Hey, you're not at the Smithsonian anymore, Toto) and the crazy crowds, it's museum-tastic.
The Spy Museum tries to both educate and immerse you in the world of a spy from your first moment in the door. I love the interactive exhibits and galleries, and I found all of the artifacts really, really interesting. I've been on the FBI tour before, but something about this museum brings the topic to life a little more.
My biggest complaint, which is shared by other reviewers, is how they handle crowd-control-- and by that, I mean they don't. For as much as I enjoyed the exhibits, the hundreds of screaming children pushing buttons, smacking signs, and touching the "do not touch" areas drove me insane. There are so many interactive areas here, but if the crowds are immense, it's hard to edge yourself in to a touch-screen or headset. Be warned that a weekend visit during the summer will make you want to pull your hair out. But if you can manage to go when it's less crowded (in DC? When is that??), the Spy Museum is a fantastic way to spend an afternoon.