In science as in pretty much any other discipline you should always weigh someones opinion by their believability / credibility. The credibility of someones opinion on something is a function of their knowledge / experience of the subject and things related to it. To first approximation most sceptics have close to zero credibility making claims about string theory, as they have not produced scientific output remotely comparable to proponents of string theory. This is simply because people like Witten tower far above most other working theoretical physicists.