I usually never upgrade to a new OS until many months later, but I had to upgrade some other pieces anyway, so I took the plunge.
I reloaded several old sessions, and everything seems to be working just fine. Actually, much better! The sessions load much faster, and performance (response time) is much better.
But absolutely make sure that your hardware has updated their drivers to work with Yosemite.
Now that I have jinxed myself, it will probably be a complete nightmare this week when I start a new session!