sgod - 2:58 am on May 1, 2013 (gmt 0)
I just finished a one-week test of the 300x600 block. Over the week, it had about a 33% lower CPM than the 300x250 block I usually have in that spot (at the top of the right sidebar). The CPC was about the same, and the CTR was lower. Most of the ads I saw there were indeed 160x600 blocks with white space to the right and left, which I found found a little ugly but not horribly so. The one text ad I saw over the week was just one link with a lot of white space under it. I didn't like that too much.
Long story short, I have taken it down. It might be worth another try in six months.