It's pretty mature, but its place within the Python ecosystem is different from JRuby's place in the Ruby ecosystem. Jython is basically an extension language for the JVM: it lets you use the familiar & comfortable Python language to script your JVM languages. Many popular Python packages won't work on Jython, either because they depend upon C extensions or because Jython is several versions behind CPython.

Ruby has much more of a bias toward writing libraries in pure Ruby, and so a greater portion of the Ruby ecosystem runs on JRuby. Also, Ruby's use-cases largely center around Rails and system administration, while Python also has large presences in scientific computing, scriptable C++ network servers, desktop GUI apps, etc, (none of which work on Jython). That makes JRuby very attractive for deploying a Rails frontend on top of JVM backend infrastructure, but the analogous situation in the Python world (Django webapp in front of say Twisted or Celery) won't even run on Jython.

I don't know about "mature", but my main frustration with jython is that it can't use a whole slew of libraries that call on native code. So a whole lot of things that happen to be important to me just aren't usable. And they aren't that exotic - things like pandas, numpy etc.

My impression is that JRuby is a lot more advanced, particularly from the point of view of parity with its "parent". That's not firsthand knowledge though: I use JRuby a lot, but Jython not for years, so I'm not plugged in to the latter. My impression is that its development kind of stalled out a few years ago, but maybe has recovered in the last few years.

JRuby performs very well, especially if you're using it in an environment like a web app, where JVM startup time isn't an issue. It's definitely better once you reach a certain threshold of usage. I wouldn't use it probably for little stuff, but if you're getting to the point where you need things like multithreading and garbage collection becomes an issue, then JRuby is a good bet.

I don't have any experience with Ruby, but I did a Jython project last year and I was rather disappointed. For a start it's still stuck on the 2.x versions of Python (I find 3.x much better, esp. for things like Unicode support). Secondly I found it to be really, really s l o w.

So while I love the concept of being able to write Python code for the JVM, I'm probably not going to be doing it again any time soon with Jython.

Jython is still hanging in there, and reached 2.7 this year. I imagine 3.x compatibility is going to require a big effort though.

But JRuby has way more momentum and a much larger userbase.

But on the .NET side, IronPython is still ticking along while IronRuby is dead.