I think part of YUI's hiatus is that she doesn't feel 'right' anymore, and that if she keeps performing as YUI, her mind/heart will break? I don't know what she was getting at in the blog entry that mentioned this, but it seems like she's ready to check out of J-Pop for a while.
https://www.facebook.com/photo.php?v...type=2&theater
English subtitled video from YUI to fans about the hiatus or something, by the way, she looks really beautiful in this video!
Mika seems to be sick or something so her vocals (and appearance imo... sometimes she looks like she has a fever!) are a bit... off her game.
Namie's vocals aren't really anything to write home about, I think, considering a lot of her songs are very 'talk-singing'.