I upgraded from 09 to 10 back when 2010 came out and it was a bit of a nightmare. Upgrade installation went smooth, but soon started getting hit with "critical error" or messages from Bitdefender to that effect. A bug report was automatically flashed on the screen and I was constantly being asked to send the error in for testing.
I got to a point in about October that I just went back to 2009. Not knocking the product, I'd just like opinions whether I should stop the 2009 updates and move up to 2010. Is 2010 now relatively stable?