you don't really need a fast aperture for shooting wildlife...you need reach, with the ISO capabilities of todays cameras you can easily shoot at f5.6-f8, and have a fast enough shutter speed...
the 100-400L is a good lens, but i wouldn't pay $950 for a used one...there are a bunch flooding the market right now, and imo the value is below that
Good observation, thanks. I always shoot at faster than 1/1000 for birds, but somehow I was not connecting to the IS performance.
Where did you see a flood of the 100-400's for under $950? I'd be interested. Out of five ads here on POTN there is only one below that.