Yes, we would benefit from the open borders and increased economic cooperation.
No, more freedoms will be taken away and it will be easier to corrupt the new government.
As long as they become part of the us.....
The US would just use its superior economic and political might to dominate the other members. You may as well annex them all and turn them into states.
It'll result in the American version of the European union and look what happened to Greece.