This puppy damn well better perform like a sun of a gun (and, according to Andy Rouse, it DOES) for that price. I'm still going to hold out for a 600mm f/4 L IS II. Its only $1200 more, for 2.25x more subject magnification.
I've been thinking about this for quite a while now (since I would be buying only by Christmas this year or spring next year) ... 600 f/4 OR a 200-400 giving me 560 f/5.6.
I've basically come to the same conclusion that a 600 f/4 will better suit my needs for shooting birds. I guess if I were shooting wildlife and/or sports, the 200-400 with the inbuilt 1.4x TC would probably make more sense.