I've now upgraded a few systems. Mostly painless I think so far. My desktop was running testing all along so it's upgrade was automatic. The first systems I upgraded were a couple of VM clients I use for SSH, not much on them and no X or GUI stuff so that went pretty painlessly.
I next tackled a laptop, and that was mostly okay except that I couldn't connect it to the WiFi after the upgrade or mount a SD card or USB stick. That turned out to be a problem in how the ck_connector was started from PAM. All things considered it's not a very visual upgrade, KDE 4 is mostly evolutionary rather than revolutionary and the same can be said of most desktop applications too.
Once the laptop was completed it was time to do my better half's desktop system. This is the second most critical system there is so it has to be right or I get complained at. As I'd already done one GUI system I was relatively happy to do this one. Couldn't get Plymouth to work but other than that it's all happy. I do rather have a long list of orphaned and old packages to clean out still.
I've now only got two systems to go, both servers, my home sever and my hosted server, both have no GUI on them so the upgrade won't be as traumatic but they do have Dovecot on them which I gather will take some effort to migrate as the old and new configuration formats are quite different. However I've plenty of time to plan for that.