Key features include embedded images, permanent URL for chat, no client to download (chat is in the web page), and easy file sharing. I’m basically thinking of it as a real time wiki or an easy to use IRC product with enhanced features.
They claim it takes 10 seconds to create a new chat, and they are correct. It is dead simple to use and has an incredibly intuitive interface. It’s a great addition to the 37 Signals product suite.