The vertical format certainly allows for more menu options.
However I must admit I was partial to the horizontal format. Why? Because the submenu is visible and clickable all the time. As an example, the "recent posts" link is one I used to click a lot. Now I have to delve into the menu.
Not a biggie, but I think you'll find you may get less people clicking on some of the previously frequently used submenu items. I've got 'recent posts' bookmarked on my toolbar though, so I guess I can still be pretty lazy.
Anyway, kudos on responding to community demand. Even if I don't agree in this particular case.