Anyone have any experience (good or bad) of implementing a Free/Open source ERP system? Which one did you choose? How did you do it? What was the final outcome?

By day I work for a company that sells and implements Syspro a commercial ERP system developed in South Africa. I work as a programmer/developer writing add-ons/mods/reports and applications that integrate with Syspro.

But I’m interested in finding out from people who’ve actually attempted implementing one of the Open Source ERP systems how it went and what problems they ran into. I’m curious to see if the Open Source model actually works from a clients point of view for ERP applications. I’m particularly interested if you implemented the product with out working with a partner company or a 3rd party doing the majority of the work yourself.

A quick search in Google brings up a rather long list of Open Source ERP systems and frameworks for building ERP systems and there are plenty of “white papers” about the implementation processes but they feel sanitized. I’m looking for real world accounts from the people who were in the trenches.?

In particular if you have any experience with the following ones (but any of the others will help):
Compiere
OpenBravo
OpenTaps
OpenERP (Used to be TinyERP)
Xtuple (PostBooks/Standard/Open Manufacturing)

Did you implement the standard system straight from the site? Or did you actually take advantage of the “Openness” and customise the application to your business?