Not seeing a whole lot of value of SfEDI over just writing a StringBuilder if you want to treat EDI like a CSV file.
http://www.X12.org (once their servers are back online) provide XSD schemas that you can generate (
http://www.liquid-technologies.com/online-xsd-to-cs-generato...) classes from that would provide full validation of an EDI document. Those classes would then allow you to serialize/deserialize from POCO to XML which most EDI providers (
http://www.spscommerce.com) support. The validation piece of that is key to making sure your documents are valid for the EDI translators that will be checking your documents prior to them getting to the supplier/customer.