I worked as a developer on Chickenfoot for about 6 months going into my sophomore year at MIT. It does share a lot of similarities with GreaseMonkey, but I've found it to be a much more powerful tool for writing quick scripts to perform simple interactions with web sites. This is largely assisted by its API of commonly used operations (some of which I wrote).

The ability to quickly package your Chickenfoot scripts into Firefox extensions also holds a lot of value.