Been using the beta since it was released - not upgraded to the full 4.0 yet. It will break some scripts and animations are handled differently now (some 3.5 settings are depreciated) but generally the process is pretty painless.
Yeah, I've been using it for a while. The legacy animation is still there for now. Many of my script errors from upgrading were very easy to pin down because unity would say "That command has been deprecated. Use this command instead."
