Love the Yveltal!!! I need one of those!!! And about the arts, I honestly like Promo Keldeo-EX, Lugia-EX and Deoxys-EX more than their original prints. And Yveltal, of course. And don't forget the Unova Dragon's EX Promos, very original. (?)
The artwork for Xerneas looks nice, but without seeing a clearer image I'm not sure if I like it more than the other EX's artwork. They seem pretty determined to have all eight antlers sticking out of the artwork box, though. Takes up a lot of room that way.
In Yveltal's case, I like the other EX's artwork more, though the two claws of the foot clutching the frame of the artwork box is a nice touch.
I always seem to like the original EX artwork better, and this is the case with both of these. Maybe my opinion will change with clearer images, however. I do like Xerneas's artwork, but it's not as good as the original.