IMO.....yes. I had the 100-400 vers 1 as my first long lens. I sold it when I had an opportunity to buy a 500.
I rented the 100-400 vers 2 for a trip to photograph Puffins on Machias Island and was truly impressed w/the improved crispness and bokeh of the vers 2.
I would buy vers 2 without hesitation if I thought I would get a lot of use....but w/the 70-200 2.8 vers 2 and the 500, I've found I don't miss the zone between 280mm thru 400mm.
I would rent the 100-400 vers 2 without hesitation if I go on another trip that would require its abilities.
Go to my Flickr site, page 3 for a few examples of the Puffin shoot; www.flickr.com/photos/wwwakehamjr/[/URL]

Bill