Just upgraded my toy to Windows 10 (MS had work to do to make it possible on tis hardware, so couldn't do it earlier).

It took a very long time to do the basic thing. Scheduling it was fun because MS interpreted the schedule as if I were in GMT+3 instead of GMT+1, so wanted to start it before I was ready (the timezone carried forwards as GMT+1 despite that). And then had to do more upgrading for the admin user, and another heap for my usual user (remaining users are rarely used, I'll fix when needed). Then I had to configure stuff where it ought to have pulled the configuration forward. And all the time message saying "this is taking longer than usual" as if MS had only tried upgrades on machines that didn't have much of anything on them.

The really annoying things were twofold: (i) cross-user permissions (where one use has access to some of another user's filestore) was all dropped and had to be recreated; that was a heavy pain and a big waster of time (cascading permissions down large trees); (ii) local services that had been on automatic start were changed to manual start (including sql server). No warning at all about either of these - found out the hard way when things just didn't work. Discovered in the course of fixing the permissions that groups had permissions that were not passed on to their members - but haven't yet tried creating a new group and assigning permissions to see if they get passed down automatically.

This is a pretty sloppy upgrade mechanism, but I guess it's better than rebuilding everything from scratch.