The focus of the MIT Museum isn't the school. It's the products of MIT: research, discovery, and good clean science. This is why it's a successful museum and a fun place to visit.
They rotate their exhibits, so every visit usually brings something novel to think about. The best part of this museum's collections is that they showcase innovation but they also encourage it. As a viewer, I was compelled to wonder "could I have made this in a better way?" This is what they're trying to accomplish, and it often works.
The museum's main gallery, on Mass Ave, is not huge. It's good for a few hours' visit. Don't plan your day around it, but make sure to see the MIT campus too (great for sightseeing) or other things in Cambridge or Boston.
Admission is free with an MIT ID, and you can take one guest for free too. Otherwise expect to pay a little, but not a lot (and nothing at all on the third Sunday of each month).