After a lot of work the Debian community has released Debian GNU/Linux 7.0 into stable. I run Testing on my desktop so it wasn't any change on that machine but on my others I therefore had to upgrade them, if I wanted them to stay on the stable/released version.
I first upgraded a couple of virtual machines and that was pretty painless. They had very little on them as they run as headless servers. I even changed the way they run within the virtual machine which should reduce their memory foot print and CPU load on the hosts system.
Next up was my better half's desktop system. Though it had a lot more installed on that the VMs it also went in fine without a problem. After that was a laptop. There were a number of issues with the laptop but they were not actually a problem with the upgrade, rather a problem hanging over from the original installation some years ago. Once the permissions were sorted out, everything started to work properly and all was well again.
The penultimate machine at home is my home server. It had a lot more server packages than the desktops on it but a lot less in the way of desktop apps to worry about. Upgrade time was very quick (I have a local APT cache) but there were issues - some of which are still over hanging. Dovecot didn't upgrade properly and required some tweaking to get it going again, though I never figured out why it didn't work... Exim also needed manual tweaking to reset it. The overhanging problem I have now is that KVM is happy to let my guest machines consume 100% of a CPU core on the host long enough so that it gets hot enough to complain... The fan on the system does kick in, but it can't cool a CPU running at 100% on a core for more than 30 minutes, so the BIOS shuts the box down.
So far I've tweaked the way KVM starts and I've set some processes onto a nicer setting but that won't help when the host isn't under much load and the KVM guest requests 100% of a host CPU. My next option is to use cgroups to restrict KVM to no more than 75% of one CPU core on the host.