It went pretty well, except you can't go direct from 3.6.0 to 4.0 - despite it saying that in the docco. You need to upgrade to 3.6.1 first, no big deal, but make sure you download that before you start.
Other than that it all went pretty easily.
Only other tip is to make sure your SQL account has sufficient access to the database. First try failed so I had to get the credentials increased. But the error message is resonably obvious.
Installing Patch 4.0 Patch 1 failed the first time round. It got stick on the PolicyMgmt.zip file. However there was a known issue fix listed for this on the release notes, which tells you exactly what to do in that case. I followed the instructions and it worked fine on the second try.
So far I like the new version, some things will take some getting used to.
I like the new dashboards feature, that makes things a hell of a lot easier than running off reports every 5 minutes when you are trying to monitor something closely.
Also, I think that some of the PCs that where not showing up in 3.6.0 have re-appeared in the directory.