Mine is horizontal.
It creates a big problem that I did not have with the PS3 though, which is that the cat sits on it while it's running.
Seriously, get your shit together Sony and make me an anti-cat console.
I wanted to hear horizontal, because that is my only choice. My cat goes after the receiver.
@demoskinos: I'm joking. The stand is for looks at best, and is best at looks. How it works is interesting though, two rubber cubes go into the central groove of the system, and rotating the two knobs on the bottom squeezes and makes them expand to the sides, giving it friction grip.
Don't have one. The slanted shape would make it easy to insert discs and USB while the system lies flat. I wouldn't want to risk it falling over, and it's small enough that it shouldn't get in the way in either position. It also makes more sense to me symmetrically.