I have both roof and porro bins. Ergonomically, I find the porro's easier to use than the roofs.
Well, it's at least part that way with me too. I usually use roofs, especially since I got the Promaster and ZEN, and especially when I have more than a binocular with me while out. But since I recently got a comparable porro in the 804 Swift Audubon, I find myself using that quite a bit. I find that going from one to the other sometimes is a nice change of pace. I recently ordered a new Swift Audubon 820ED, just to have a top end porro around.
By and large, I do like the feel of the porro prism binocular. But, I am not small, and I have pretty large hands.
then I must be a stubborn roof convert since many years ago.