Utada's recent performance of Goodbye Happiness is really well-done, in my opinion. I have always thought that she has a very wonderful voice.
http://www.youtube.com/watch?v=sFkvLN9pt-k Here is the video I was talking about.
Kremlin Dusk is a very challenging song! (not to mention it turned out the stage was made out of cedar wood, which she is entirely allergic to.)
I think the truth about this all, like mizuki-7 stated, is that it's a matter of taste. Utada's been very well trained since she was a child. I think that she has a fragile voice and should not push it too hard. So, the first portion of Utada United was a good idea, but I think it proved that she needs to not take on so much in one night! Some singers are just made that way.
I'm sorry for my Utada Hikaru rant just now. >.< I actually don't listen to much of her music anymore, but she was the singer that got me into Japanese music. So... I guess she inspires me a bit?
As for Ayumi Hamasaki. I loooooooved her voice when she first started off. I know a lot of people don't care for her old voice, but I find it to be so unique and amazing.
I'm starting to get used to her newer voice, but it's taking some time for me. I love her music so much, though!
Anyway. I definitely agree that singers are truly more exciting when their lives are better than their recordings. Though, being a nu metal fan, I have to learn to live with the fact that screamers can only do certain things with their voice when live... >.> (because of the preparation they have to do to hit certain notes...)