I investigated OpenERP, and actually used it for a few months something like two years ago. It felt nice indeed, and I particularly liked the ability to set default values for many form fields (a global default value overridden by a client-specific default value, for instance).
However, I have to disagree on the "true free software". At least at the time I investigated, there was no procedure for upgrades across major versions apart from "hire us", and the website was quite bluntly announcing that bug reports were very likely to be ignored unless they included the identification of a customer support contract, and patches would be similarly laughed at.
Did that change in the last two years?
(If anyone cares: I switched to Tryton, which happens to be an OpenERP fork.)