As for my opinion, I am very against SOPA, PIPA, and ACTA. Not for the purpose they are created for, but for the consequences they cause. Stopping piracy is something we need to look into, its a growing problem and it needs to be at least slowed down, as I find it hard if not impossible to stop. Furthermore, the internet itself does need regulation. With cyber attacks becoming a major national security threat, and groups such as anonymous threatening to shut down the internet, the reality of cyber crime is once again coming to the eyes of governments everywhere. Its always been an annoyance, but it hasn't been as much of a problem since the days of the Chaos computer club, cDc, and many other hacker groups of the 70-90s. Now it has come back, and the potential of threat is much greater. But as much as I would like to discuss that, you're asking about SOPA.
SOPA is a fairly good bill when taking a simple look at it. A bill to prevent stealing other peoples work. How bad can that be? However when taking a look at how they intend to prevent it, a problem becomes much clearer. First of all, a big problem thats attracted much attention for the bill is the DNS blocking. Under SOPA, someone could claim a website holds copyrighted material, and shut it down. Forcing it into a guilty-until-proven-innocent type of judicial system. Aside from that, it also could be abused to gain an advantage, as shutting down a competitor during cyber monday (monday after black friday for internet sales). Giving it a huge advantage in sales. Also it could be used for political gains. Shutting down a competitor in an election, or even more, shutting down activists against the government. Had SOPA passed prior to 2010, the government very easily could have used it to prevent wikileaks from releasing the diplomatic cables, and thus silencing their cause.
But its not just censorship that is the problem when it comes to DNS blocking, such a drastic change to the DNS infrastructure could damage it, stunt the growth of the internet, and bring possible potential for a 0-day cyber attack.
The potential for abuse brought upon by SOPA could prevent many forms of entertainment from existing on the internet anymore. The internet would likely fall back 20 years and be much more a tool for information rather than entertainment.
All of these reasons, plus more I'm not mentioning are why I disapprove of SOPA.