I don't understand all you people saying that you prefer tip-up because the knife is already in position and you don't have to adjust it.
When I pull a tip-up knife out of my pocket, I'm usually only able to pinch the little portion that is sticking out of my pocket, once I have the knife out, I have to shuffle my hand up the knife into proper position.
With a tip-down knife, I pinch the part sticking out of my pocket and as I lift the knife my palm rotates down into the proper position.
I don't think tip-up offers a speed advantage at all, I would argue the opposite.
That being said, I have many of each style and to be honest, it really makes no difference to me. The only time it's an issue for me is when I forget the orientation of the knife I am carrying that day and expect it to be in the opposite position.