I used it once.. and never again too lol well partly because I can't wear any hat/face accessory while wearing the mask, mostly because it just looks weird hahah
I got it he first time or second I went there, just so I would have the option if I wanted. It doesn't take up space really, but I don't typically wear it, sometimes because I tried to get it to look like me, but it does make the head look strange on the body a little I think.
I've seen other people wear it and make it look alright, though I personally have never made one in AC:NL (I made one in City Folk I think and it was terrifying, but my Mii-making skills were pretty horrific back then). It really depends on the Mii, I think. Maybe I'll make one tomorrow to see how it looks.