I don't have Shampoodles yet so I can't use a Mii face... ;A; I like it sometimes! It can look pretty cute but then you have to forego all face and head accessories and that's pretty boring. Also, Miis can't have fun hair colors so that's a bummer too.
I don't like using the Mii face. It looks out of place and your head looks so big. I'm not a fan putting in mask in general so probably that's another reason.