You can call any method, inspect objects (using props), see where DOM objects are (using blink), and the killer feature, in my view, is the autocompletion using the tab key! It also features a command history, which is very convenient to try variations of the same call.
And if that wasn’t enough, you can also use the script as a bookmarklet. There is a Firefox version, and an Internet Explorer version available from Curiosity is bliss (ported to IE by Julien Couvreur).
You should try the script for yourself.
This is a great tool to debug, or even try a few things quickly and this will be a good complement to the rails console. It sure beats having to reload the page!