The only rough element I noticed was that clicking in the shell window doesn’t take focus unless you click next to the shell prompt, at least under Chrome that is. So remember that if you find it unresponsive.
For more experienced developers, the shell has a number of interesting elements. The browser shell talks to a background mongodb instance with a Flash based front end. That backend handles sandboxing of the various user sessions connected to it and ensures that the code being entered doesn’t choke the server. 10Gen have released the source to the shell, under an Apache 2.0 licence, on GitHub.