I've wanted to move from a manual focus lens (Tair 300mm f4.5) to an AF lens, because I was getting tired of shooting wildlife with manual focus as I was missing a lot of shots (especially moving targets).
There were only 3 options for me: 100-400 L, 300 F4L IS and 400 5.6L
The zoom was out of the question, since I would be shooting 90% of the time at the long end (400mm), and then the battle was on between 300 F4L IS and 400 5.6L.
The main advantages of the 300 were F4 and IS. I liked the fact that it had 1.5m MFD,but soon I've learned that that MFD is not a deal breaker (For me) with the 400 (which has 3.5m).
I finally decided on the 400mm because I had already used a 300mm for a while,and if I'm upgrading to a new lens, besides the AF, I just wanted some extra reach.
I've got the lens for 2 weeks and used it on a 5Dc, 5D3 and 50D.
On the 5D3 it works superb, considering the high ISO capability of the camera to help the slow 5.6 aperture of the lens. Not to mention matching the advanced AF system of the 5D3 with the fast AF speed of the 400 5.6 . If you're shooting BIF or distant targets, set the lens AF to 8.5-infinite, and the AF is lightning fast.
Sure, a lot of people would have wanted IS on the lens,but then the price wouldn't have been the same, the weight wouldn't have been the same, etc.
To me, the only disappointment is the lack of some decent weather sealing,since this is thought out as a wildlife/sports lens.
F5.6 is annoying,especially on the 5D, as I find myself shooting ISO 800+ almost all the time,with this lens, but the results are very good. I can't complain on the aperture,since I knew what I was getting into.
For somebody who wants extra reach from a telephoto (wildlife,etc) I can't think of a better lens (on a budget) than the 400mm 5.6L
For an extra 100mm and an F stop (500 MM F4 IS) you have to pay 5-6 times the price and I don't think it's worth it.
I also browsed the web for reviews of the new Tamron 150-600, but it didn't convince me.
It's a shame that Canon doesn't have a cheaper alternative for the 500mm+ FL