It is using OpenACS Core, but that's about it. It has it's own Project Management, Invoicing (which is only GPL in part), CRM System and so on. The reason for us not to use PO for one project were rooted in the fact that most of PO is a rewrite of the ACS Intranet and is therefore a little bit harder to maintain from a developers point of view. Furthermore it blocks you from enhancements in Contacts, Project Manager, Invoices or the new Accounting Software written by Torben et.al (datamodel is incompatible). On the other hand you get a product that does what you want (hopefully) out of the box, albeit with a somewhat interesting licensing model (depending on whether you want to deploy the software on your own for yourself or for your customers).
