since we are a small B2B company, the costs have been very important for us - so we looked into several solutions for other open source systems and found the integration with OpenERP mostly good.
When you talk about ERP, you might be prepared to spend a lot of time or money on setup, configuring, customizing - unfortunately that’s the same at OpenERP..
Positive: You can have a system with Magento integration up and running within a couple of hours.
Negative: Very little well documented things and the help from the forums is even worse than here.
Some things seem just to not work at all. Like: Customer data sync. I cannot get new data into the other system, independent where I updated the record. That’s a bit strange - but maybe I just use it in a wrong way?
OpenERP on the other hand offers many modules which work quite good, once one understood how to work with them.
It took me quite a while to figure out how email marketing works, but now it seems to be good.
About production and fullfillment: Unfortunately I didn’t look into this yet but OpenERP can do it (see the youtube demos).
I’m still at the very beginning (setup 3 weeks ago and casually working on it) but it seems to be the right direction for us, eventhough it will take quite a while.