Frank Scavo recently posted an interesting analysis of a succesful Compiere Implementation story in Petronix; a California, USA based automotive distributor, you can see Frank's post here.
He comes up with a conclusion that major OSS ERP advantages come from direct savings in the upfront costs of the project due to the lack of a license cost, for one thing, plus he mentions other several major advantages such as flexibility and the abscence of vendor lock-in.
On the other hand he recognizes that OSS ERP is not for everyone to adopt. A Careful thinking of the pros and cons of such a decision must be considered prior to choosing to either go for the commercial or the open source alternative.
I still think that in the long race, Compiere has a major handicap against commercial ERP because it is easier to determine net value creation derived from a lower cost of ownership, in the first place.
The downside is, we are still yet to see critical mass adoption of Open Source ERP, that would give us more credibility and 'pedigree' (as Don Klaiss mentioned it). I would also like to see a wider and bigger community of Users, Partners, Customers and Developers revolving around this new Enterprise application paradigm.