I actually use it vertically most of the time for the same reason. Typing on the keyboard in landscape just seems too cramped. I use the split keyboard in portrait mode and type with my thumbs. Works very well.I do a lot of typing on mine, so it's almost always Horizontal (landscape).
The keyboard is too small in portrait to really type at any speed.
I have an external Apple bluetooth keyboard, but I don't use it very often. It's inconvenient to drag along with me.
About the only time I flip into portrait mode is to read a book.