Even though he retired from the bench, former Supreme Court Justice John Paul Stevens, 92, is still giving it good.  He's an outspoken critic of Citizens United, saying yesterday:

"If the First Amendment does not protect the right of a graduate of Harvard Law School to spend his own money to support the candidate of his choice simply because his Canadian citizenship deprives him of the right to participate in our elections, the fact that corporations may be owned or controlled by Canadians — indeed, in my judgment, the fact that corporations have no right to vote — should give Congress the power to exclude them from direct participation in the electoral process" 

The speech Stevens gave yesterday is a good read (PDF)