I have an awesome Mew decal on mine!
I have one of the black New 3DS XL's, and I have it bare. I've always like my electronics without protection, I just love the way they feel. The new 3DS has a kinda iridescent effect too, it looks pretty fantastic.
I had a custom decal put on my red 3DS XLwith a nice Totoro mural on the front, this Studio Ghibli logo in red and black on the back and sliced this image of No Face to be put on the inside. Unfortunately at some point the front got nicked and now there's a small gash in the whole thing.
I had a minimalistic pokeball decal on mine for the longest time.