Well opening button flys can indeed be faster, assuming you open your belt and top-most button aswell. But closing it again is definitely not faster than zipper, no way. Plus, I don't like undoing my belt and buttons at the urinal, I always have a feeling that my pants are about to slip down :P
Yeah I guess that's true about the "bending up" at the crotch area when sitting down. I admit that's one disadvantage of zipper flys, though for me it's not really a huge deal. It just looks "suspicious", but it's not uncomfortable or anything.
Diesel are not bad, and a lot of their models have button fly.