I upgraded my home machines and my work machine (CentOS 5.2) to Flash 10 recently through adobe’s repo. The upgrade process itself went smoothly – even on my x86_64 machine, no extra steps. Just came in as a PackageKit update, restarted firefox and there it was.
It went steadily down hill from there. From all appearances at first it seemed to act like flash 9 on sites like youtube.com, homestarrunner.com, and wordpress.com, but it seems to want to crack under use. It would break in the middle of videos. Charts that were based on flash would end up somehow randomly off-center and only the left edge of it would be shown.
If you were thinking about upgrading from flash 9, I would say, don’t. Wait for the next patch release and hope that one is more stable.
To read another opinion about it, check out this article on ComputerWorld.com