My complaint has always been the lack of available open source code for HL7 v3. Microsoft has been great in recent years by releasing sample .NET code such as the "Application Blocks" (see for example) and various reference architectures. They have now released code for a working sample HL7 v3 application using BizTalk. I'm not really crazy about the undue complexity in implementing HL7 v3, but I hope Microsoft can make it more practical to implement using their platforms such as BizTalk.
Go take a look at Roberto Ruggeri's blog item: Getting Started with HL7 Version 3 and BizTalk Server 2006 (R2) for more information.
Here is the link to the sample code. Here is the whitepaper on getting started with BizTalk and HL7 v3.