I recently upgraded my Server 2012 'Workstation' to 2012 R2 RTM.
The system is mainly used for testing changes etc and is an old rack mounted Dell 610 - 48gb RAM with a solid state boot disk and filled with as many 7.2K cheap/large capacity disks as it will hold in RAID 6 to give reasonable disk IO and large capacity. It also has a 4 port Intel network card with a couple of the ports teamed together mainly to test that the new MS NIC teaming works as intended. The NIC team has worked flawlessly for a year laying to rest the Server 2003 scalable networking debacle.
Following upgrade from Server 2012 to Server 2012 R2 I couldn't access it via remote desktop so I went in through the Dell Remote Access Card (DRAC) and looked at the console. After a few minutes poking about it I had worked out that the NIC teaming had been removed and the network set to DHCP. I was able to connect to the DHCP IP address and sorted it out from there. I would say this behavior is very unfortunate in enterprise computing. Normally I would have said it was a one off but the same thing happened a few years ago when upgrading some Server 2008 to Server 2008 R2 Dell Servers, which lost their network card settings during upgrade. If the server is remote and you don't have a remote access tool like the DRAC then you could be in a lot of trouble. Worth watching out for.
I was looking to change the Windows Azure Backup Policy on the machine after upgrade but it wouldn't change, and kept coming up with a generic error after a timeout period. I deleted it and recreated it from scratch and it is now running fine. I cannot imagine how that backup software changed between versions as it was a manual download from the internet but there you go.
Apart from these upgrade errors the Server 2012 R2 release is looking good with many new features.
