I tried the 100-400L earlier this summer and it appears to be a decent lens
other lenses that look good on paper (price and features) are the 300F4 and the 400F5.6.
I am also considering the 70-200f2.8 ISII and a 2x converter
Honestly, all are very good options. My Flickr Birds
set is mostly with the 7D + 100-400mm, some with the 70-200 II + 1.4x or 2x.
The difference between those two in terms of IQ is minimal (slightly in favor of the 100-400 at 400mm). If you plan to do a lot of shooting with the 70-200 II bare, and only occasionally use the 2x, that's a great option. If you will routinely need the long end of the range, I'd get it in a native lens.
The 400/5.6 long focal length and lack of IS mean you need a high shutter speed to use the lens, it is great for birds in flight with the fast AF. The 100-400mm is a great, and versatile lens - among the options on the table, I'd go with that one. Do keep in mind that with the 5DIII, once the new FW comes out next year you'll be able to AF at f/8 just like the 1D X. The shot below was with the 1D X, 100-400L + 1.4xIII at 1/320 s, f/8, ISO 10000.