I got to use this alternative design on my second Facebook account that I used for app development, while my personal account didn't have it enabled. I really disliked like the new sidebar design. The concept was similar to what GMail has done lately, with text links replaced by only graphical icons. I found it really difficult to remember what each icon linked to, and I'd have to go through and hover over each icon one by one.
My theory (which I think has as much evidence to back it up as Dustin's) is that if the feed performed better in this design, it was because the poorly designed menu made it more difficult to navigate the rest of the site!