Avantages
Great intellectual atmosphere. Nice resources. Opportunities to meet and collaborate with lots of smart, motivated people. They are at least making an effort to improve relations with their postdoc community.
Inconvénients
Though there are really great resources here, they are often owned by particular labs; so you may not be able to have access to them. Postdocs are in a strange category of personnel here, and they don't quite know how to deal with us. They don't give postdocs access to the careers office, and many PIs don't have time for or knowledge of the job market (even inside academia), which doesn't help postdocs to move on. Your experience will largely be dictated by your PI, your project(s), how hard you work, and luck, just like everywhere else. Don't assume, as many do, that because it is MIT it will automatically be better than a postdoc position elsewhere.