Continuous integration and automation. Comparing the common tools

You can only claim that your testing project is fully automated if human effort is completely removed from the testing process. Ideally human involvement should be limited to viewing automated testing reports that he regularly gets to his e-mail.

Achieving this is possible only by using continuous integration systems. So which tool is better? Why? Is this choice so important or maybe you can just pick the random one?

Let's compare the most popular Java-oriented CI tools and decide!