I tried using it a month or so back and had a great time.
Caveats:
* You have to use a special patched version of virtualenv.
* Numpy is unavailable for Pypy. For various reasons the pypy folks are just going to have to re-implement it.
The lack of numpy turned out to be a deal breaker for me in the end. I look forward to using it again once there version of numpy is ready for consumption.
For folks that want to test their projects against various version of CPython, PyPy, Jython, etc., check out tox: http://tox.testrun.org/