I've found it quite useful for seeing what stuff is installed on a machine, and to quickly look up packages from pypi.
Here's some of the things you can do with it:
yolk -H twisded
Launches your web browser at Twisted's home page
yolk -M Paste 1.0
Show all the metadata for Paste version 1.0
yolk -M Paste
Show all the metadata for each version of Paste listed on PyPi?
yolk -D cheesecake
Show all URL's for cheesecake packages you can download
yolk -T source -D cheesecake
Show only source code releases for cheesecake