So far, I've found a couple of useful features which I've listed below:
1) Continuously run projects overnight, 7 days a week.
2) Make project management visually appealing.
3) Identify changes/modifications/defects.
4) Integrate a test tool with it which runs tests continuously.
5) Export test result data to visually appealing web page.
6) Show project status graphs to customers, project managers, developers.
7) Open Source
The features I personally use on a daily basis is the test tool integration feature. The CI is set up to run all our tests throughout the night.
Our CI gui has a link to various pages.
Examples: We've pages of the results of all the tests displaying test passes/failure/coverage deficiencies/whatever we tell it to display.
We have graphs displaying:
1) Open Tasks
2) Compiler Warnings
3) Test Results
We have links to changes that was made that day to the source/tests and by who it was made; which files were affected; which bugs were logged/closed.
Anything from bug references, test results, warning outputs can be accessed from the CI.
The amount of pages/features/graphs/projects you can add is really limitless. Just need to write the java plugins yourself.
Go check it out for yourself for a more in depth and accurate discussion on the topic.
Here is a link to one of many CIs:
P.S. **!! Please Vote !!**