I've used SlideIt, Shapewriter, and Swype and am in the beta test group for Swype. As I've noted on other threads on this subject, I don't think it's possible to evaluate any of these input methods without committing to each for a relatively extended period. What seems great and magical on day one is sometimes annoying by the end of a week. And vice versa. None of the three is a perfect solution, imo. Some work better in portrait mode and suffer when used in landscape mode.
Furthermore, if human beings can adapt to the bizarre demands of a qwerty keyboard, they can adapt to almost anything. And usually claim that it's "natural" once they've done so.
Having said that, I find Swype to be the best of the three "swiping" products. IMO, it's the most accurate and the editing features (critical) are the most advanced and useful. I seldom type more than a couple of paragraphs on my phone so I can't say which approach is better for long emails or texts. And even if one approach were clearly superior for that purpose, I'd still compose prose on my desktop 95% of the time.
As for what I'd pay for it, I have to consider that entering text on the Droid is one of the most common interactions I have with the device. And if I'm willing to pay $10-$20 for an app that enables me to edit Word documents, it seems only reasonable to pay at least that much for an app that makes it easy to do so.