And we're moderately content with it. It has quirks, and it's often complicated.
What I really don't like are the upgrades. As it happens, OpenERP is a collection of plugins. Every plugin defines the database-fields in it's forms, which are inserted/generated upon installation of the plugin. Which means dozens of plugins modify the same tables.
This makes migrating, even to a never version of openerp, really unpleasant.