With 6.5 released we have started upgrading machines. Our initial upgrades were fairly straightforward and although the install of the Xentools was very slow and required numerous restarts it completed fairly well.
We have had issues with 2012 server running Exchange 2013 - insofar that we've lost the network adapter and can't get it back. The solution seems to have been to clone the VM and then remove and add the adaptor back in. The restart took a very long time and there were issues with the Xentools install not working properly and difficulties with getting it to upgrade. We ended up removing it and reinstalling it however that wasn't really optimal and we blew our outage window but quite a margin.
Before upgrading Xentools definitely take a snapshot of the VM if it's running Exchange. As we do more I will continue to update this site with more information.