Personally, I've only ever used CruiseControl and CruiseControl.Net. The reason for this has to do with economics. They are reasonably stable and once you set them up, there really is little you need to do to maintain it. The user community is usually very helpful, and it can be extended to your needs.

That said, there are a couple commercial offerings available that I am aware of (one by JetBrains, the other by Atlassian) which offer a better set up experience and commercial support. I've been meaning to try these offerings but really haven't had a chance yet.

CI tools have a more important role to play with compiled languages than interpreted languages, but that isn't to say that the CI tool is wasted on interpreted languages. When you have several projects that depend on each other, and you want to make sure a change doesn't accidentally break it's dependencies--CI tools are invaluable.

There's three general classes of problems that CI tools can help you catch:

Compile errors -- if the signature of a class changes in a way that breaks dependencies, it's best to know about it before the waining hours of a deliverable. Logic errors -- if the behavior of a class changes in a way that breaks dependencies, it's best to know about it early. This has to be checked by some sort of automated testing, most commonly unit testing. Acceptance Testing -- if you have an automated suite of tests to run on the finished product, it's best to run them often.

Interpreted languages are not compiled, so there are no compilation errors to catch. However, the other two problems are common enough that CI tools are useful for projects in Ruby/Python/Perl/etc.

The key word in both the logic errors and acceptance testing points is "automated" testing. If you don't have a suite of tests a machine can run, then you really are missing the greater benefits of CI tools. Automated suites can be built up with time, so you can start small.

Edit

See this nice chart for feature comparisons of a large number of CI Tools (many of which I didn't know about):

http://confluence.public.thoughtworks.org/display/CC/CI+Feature+Matrix