Ok, I'll say it: I'd pay for Twitter, but only if it was free for everyone else. Let me explain.
In my line of work, it's by far the most useful social tool for delivering me interesting and relevant information. I hear similar things from other journalists and tech-lovers. Yet a USC Annenberg School of Communication survey released yesterday found that 0 percent of respondents said they'd pay for Twitter [pdf]. The obvious interpretation of the study is that people, even Twitter users, don't see a lot of value in the service.
But I don't think that's quite it. It's not that Twitter is worthless, but rather that in trying to charge for its value, you lose it. (The Twitter Uncertainty Principle?) A paywalled Twitter would destroy the healthy information ecosystem that's grown up around it. Users realize that Twitter has to be free to be Twitter, so of course they won't say they'd pay for it. The sum is worth a lot, but if and only if it's the sum.