Well, when I was doing yoga regularly for 2+ years, I never even saw any of these poses in a class. And most of the classes were intermediate or mixed level.
My balance sucks, and while it has improved tons, from cycling, yoga, and x country skiing, I would not attempt any of these. There seems to be a lot of these poses around on the web lately. I think it's kind of funny, since in my experience, most people can't even hold tree pose for more than 2 seconds.