We did a full re-write last summer.

It was absolutely worth it and has since allowed us to iterate at a much higher speed. It was a bit terrifying to be at a feature standstill.

Our re-write was from Java(struts2) + BDB to python(pylons) + MongoDB.

If anybody is interested we are giving a talk at PyCon.

MongoDB + Pylons at https://catch.com: Scalable Web Apps with Python and NoSQL: http://us.pycon.org/2011/schedule/sessions/131/