Biggest benefit of portrait is you can instantly subconsciously see more about how long the article is. This is really most useful in the cases when the end of the article is visible on the first screenful in portrait mode and it wouldn't have been visible in landscape mode. This gives one a pleasurable immediate knowledge of how long the article is, how soon it's going to wrap up, etc. It was also quite nice in the Google Reader UI where you could get an idea of what was coming next.

But that comes at several costs. One is that text is less frequently at your optimal reading height. Often the text will be too high or too low for comfortable reading. So you end up having to either scroll frequently with a mouse wheel or touchpad or say scroll say half page at a time with the keyboard.

Having text at a non-optimal height is also annoying when using text editors in this portrait mode. Sure you can see more, but more often what you want to edit or read closely will be too high or too low unless you use C-l (emacs binding) frequently to center it.

Also, for those people who haven't given up their mouse yet, it can be very annoying to move the mouse this much distance. If you have part of a UI at the top (say tabs, menus, toolbar etc) and then part of a UI at the bottom (say find box or start menu or media player controls or) then moving the mouse that distance can be an annoyance.

And even if you don't use a mouse, then you will now have crucial parts of the UI (like minibuffer or mode-line or gnuscreen status line) at the very bottom of the screen in an uncomfortable-to-read position.

Overall from my experience I loved it for getting an overview of webpages, for Google Reader, or seeing entire pdf pages at once, but not much else.