It looks very good which, in my eyes, means very intuitive and ergonomic, straight to the task, something one is entitled to expect from a team of web design and usability experts. It's also refreshingly Mac-friendly with calendars hookups to iCal (similar hooks with Entourage would be perfect) and works well with all sorts of browsers on Mac and PC (not a plus but a necessity for many web design professionals).
The ASP service is free for one project and provides pay-for plans depending on the number of active projects hosted (you are not taxed at every corner such as per the number of participants, like with other products). It takes about five minutes to get a project setup, and all the plans have a 30-days trial period, so anyone can make their opinion before buying. There will be an installed version in the pipe, which can be hosted behind a company firewall and requires OS X/Linux/Freebsd, MYSQL, and Ruby. Basecamp will not be available for install on Microsoft servers (do I see a pattern here?). The costs of the installed version have not been disclosed yet.
Time, and use, will tell how good Basecamp is, but I'm already confident that now there a working cross-platform replacement for the mono-platform Microsoft SharePoint (and the defunct Macromedia SiteSpring software which was also quite Windows centric).