Hi Joe,
SAP PI recently launched B2B Add on (needs extra License Cost) to perform B2B communications. Before that SAP PI has the options like Seeburger / Advantco....these 3rd party (Different companies which also need extra licensing cost) adapters to achieve B2B requirements and these adapters are compatible with SAP PI. In present either you can go for SAP B2B Add on / these 3rd party products.
Seeburger is already a proven and very much stable solution for B2B and EDI communications. It has presence of more than 25 years in this platform. Seeburger also works with SAP in a very good way.
I have used B2B Add on and also found good and no doubt ready to compete with other leaders, but as it is new may be couple of changes / upgrades will be done by SAP.