I didn't mind Sam's performance actually.
I sat there knowing that it was coming up and having seen Kerry Ellis perform this live, and I couldn't see how she'd manage it. I've seen Cassidy Janson perform this in the West End too, and I don't think even she managed to give a great performance of the song even though she has been training to sing it (Elphaba U/S).
When the clips of Idina came on I wanted to shout at the screen. Yes idina originated the role (not counting SJB!

) but she really did have pitch problems with Defying Gravity (as shown by other posters) so I really couldn';t understand why Idina should be the one to help her sing it. Over rated in my opinion. Idina is much better in Rent (but that;s a different story)
So Sam started singing. badly. But it's such a difficult start that i kind of waited to judge and she really suprised me. She was no Kerry Ellis but I think she probably sang this as well as CJ and although I love the growling at the top of the song, Sam's clearness was actually lovely to hear.
It wasn't the greatest performance ever but I have to give it to Sam that she chose a MT song and sang it fairly well. Would hate to see her in the role in London, save that for the pros!
(and can I just cover this post with; it's all my opinion! don't shout at me!

)