I was at the show where these clips were recorded at. For sure these digicam/cell phone videos do not make justice for her. I thought she did a great job especially with Ever Dream, Sleeping Sun and Sacrament of Wilderness. When you were there, her voice really filled the whole venue even when she seemed to taking it more easily and not pushing her voice to the limits. Naturally she did a very good job with the new songs too and sounded pretty much the same as on the album, although maybe not sounding as nasal on some stuff (the album was recorded months ago so I guess it could make a difference).
So yeah, of course with a different range she has to do some of the old stuff differently and they have re-recorded the backup stuff to fit her voice better too. For me, on most of the old songs it worked very well and I am now looking forward to see them again with her more than I did during the last couple of years of Tarja era. She seems to be more human on stage too (as in not performing as "robotic" as Tarja did every show, not that I didn't like her live though). Also, seeing how the guys love to play with her makes it feel like more of a band now and not "Nightwish featuring Tarja". Pretty sure most people will be impressed when actually seeing her live on the US tour even if they didn't exactly love the youtube vids. Beginning of a new excellent era, if you ask me.