Thx for the post, Wengler. I'm shooting with a 7D. I think I've settled on the 100-400L but will still rent it to be sure. It holds its value well so if something better comes along in the future I may still get a lot back from selling it. Yes I'm looking for used lenses as well. It looks like going price is $1000-1200.
I have a Sigma 120-400 OS that I use on my 60D (which is very similar to the Sigma 150-500). I've found it to be very sharp and I'm quite happy with it. I got it used (but looked new) from someone local.
I settled on the Sigma because I'm not very serious about shooting long... I just wanted to have something available when the opportunity came up to use it. If I was regularly shooting wildlife, then I would probably be looking at the L, just because I think it is a more reliable choice.
But those are all good lenses, and I think you would happy with any of them in the end.