We have been using OpenERP for a year and a half now. It has had some issues, but is overall most excellent. It also has a vibrant developer community, and is true free software, not free core stuff. You can see the wide range of modules here, for the most part GPL: http://apps.openerp.com/
OpenERP is even being taught in an MIS class at CU Boulder, in LWN's neck of the woods. This is the second year the course has been offered.
Python, Postgres, truly open, native GTK client, web client, large community, easy records import/export via CSV, years of open development, what's not to like? :)