I am using it and, yes, many plugins are not 3.0-ready. However, you can go to the plugin site and typically find a simliar plugin. And, sometimes, the website has an update to that plugin for 3.0 and it doesn’t seem to pick it up during the auto-plugin-upgrade at start.
I love the new firefox, but one thing I noticed is that some AJAX-enabled apps are not working. Not sure why, though, as the same apps will work in the earlier versions of FF.