I oppose SOPA and PIPA. Here's a good summary why: http://nielsenhayden.com/503_sopa.html.
The argument I included in my notes to my Congressional reps was this:
Imagine that Walmart and Target got up in front of Congress and argued that since people shoplift, they should be able to stop cars and trucks randomly on the streets and highways looking for stolen goods, and to take away drivers licenses at will. They would (I hope!) be laughed out of Washington. But that's effectively what the RIAA and the MPAA are demanding with SOPA and PIPA -- except that it's not just roads, it's the Internet they're demanding police powers over (with no oversight or appeal).
The Internet is the greatest force for free speech since the printing press. It is the medium through which a huge fraction of our news, entertainment, political discussion, and, indeed, of our economy flows.
Intellectual property piracy may be a problem -- although all independent research says that the RIAA and MPAA are exaggerating the problem by several orders of magnitude. Giving the five Hollywood studios, four multinational record labels, and six global publishers the keys to the Internet is an awful idea.