Using it is simple: you copy email@example.com on an e-mail, and it creates a Web-based "mail space" for the thread. The system parses out dates and locations as downloadable calendar items and map links. It also creates catalogs of media items (pictures and videos) and files that are attached to messages.
There's nothing in a Cc:Betty mail space that isn't stored inherently in the e-mail messages themselves, but the system's cataloging and feature extraction makes mail threads more useful and much easier to navigate if and when they get convoluted with a lot of people chiming in.
Unlike the mail client add-ons (in particular, Xobni), Cc:Betty works with any e-mail system. Also unlike Xobni, it's useful to everyone in a conversation, not just those who have installed the Xobni add-on.
Using Cc:Betty reminds me of two different experiences: 1) Bug tracking system Bugzilla, which is a product some companies (like mine) use to track conversations around software development. 2) Tripit, the travel planning assistant that works like Cc:Betty: you cc it on your travel plans and it creates a useful record of all the related content for you.
Cc:Betty is missing a critical feature right now: it has no search function. That's supposed to be coming in about a week.
The service is free at the moment. Signing up is automatic if you copy a message to firstname.lastname@example.org. A paid, premium version with additional features is coming.