Don't go here thinking you can conquer this museum in an hour or two- its huge!
The building alone is impressive, add in the various exhibits of instruments from all over the world, and I can see why Tony Bennett deems the MIM as the "best museum I've ever seen"
The MIM does a fantastic job of presenting all of the instrument history and information in a way thats easily digestible for a musical novice like myself. Definitely reserve 3 or 4 hours to see and experience it all. They even have a nice cafe to grab lunch after you've worked up an appetite from all the exploring.
Lastly, DO NOT FORGET to grab a headset. The headset changes the whole experience and allows you to be transported to a different time and place through the different instrument's sounds as you wander the exhibits. Rock on, MIM.