Mark Kissel , Jul 21, 2011; 07:43 p.m.
I don't have the 70-300L, I do have the 100-400L (and I like it a lot, but it is a tad soft around 400mm). I'm glad you asked this question as I would be interested in hearing feedback as well. I'm sure you've probably seen the reviews over at the DP. Looks like a pretty favorable review.