There have been some interesting musings on HL7 v3 recently on this site: http://www.healthintersections.com.au/?p=476#comment-574
. I don't think you can say whether HL7 v3 is successful or not just like that - there's a gradation obviously - but the comments are really interesting.
How about this for an unconventional idea though? If two parties agree to use exactly the same HL7 messages - in other words the over the wire format for a given interaction is agreed completely - then both sides have a template with placeholders for variables. So in this case really all you need to send is the delta. The full sent message and the expected full received message are the same except for the items of information. So if we consider that full message a template, then one could mark out where variables a,b,c etc would be expected. If you know that, then you might be able to send just the variables and not all the surrounding message infrastructure since the receiving party already has it.
The NHS in the UK is considering abandoning its centralised storage of eHealth records. See the Parliamentary report. There's a news article here about it.
