We’ve been using the .Net connector for a couple of years. I understand that Microsoft uses it as the basis for their Biztalk SAP Adapter as well. It works for us. If you have access to the SAP Developer Network (www.sdn.sap.com), that’s where SAP provides a lot of support and information about how to use it. Also a lot of entries for it on the SAP Service Marketplace.
We’ve been using NCo to to build custom web-apps for clients for a couple of years now.