It's a great tool for trapping politicians, although of course it could backfire. Change in a platform can leave a politician open to justifiable attack, but some can be good. (See "Flip-Flop Flap" in the current New Yorker magazine.)
Beyond the gotcha value, there are other very useful applications for this service. You can use it to monitor prices on a product page. You can keep an eye on a competitor's site for changes relevant to your business, or for additions to their news page (although Google alerts can also work for that). Writers or commenters on blogs can use the service to see what changes publishers make to their stories after initial publication.
You can see changes in pages you're tracking in a wiki-like list, and you can display two versions of a page side-by-side, with the differences highlighted. The system can also track all the pages linked to from your target.
You can track two pages for free. Paid versions range from $16 to $499 a month and give you more URLs and storage space, and more control over filters.
The service will send you daily e-mails with updates. There's no RSS feed yet.
Versionista is reminiscent of Iterasi (review), an archival system for Web pages that will at some point add a feature to automatically capture pages into an archive. But if you're more interested in tracking changes than building a library, Versionista is a cleaner solution.