I tried it with the 3 and 1V on an bird photographer workshop. Wow. this lens is great. lighter than the old version and with an fast AF.
Are you refering to the Canon 500mm f/4 IS ver I? It is my understanding that the 500mm f/4 IS ver II is not available for purchase yet. If you want to buy the ver I of this lens, purchasing new is very unlikely and finding one used is also difficult and expensive.
The 500mm f/4 IS with the 1.4x extender (III or II) will autofocus with the 5d III and the 1D X and will give you a 700mm FL and a more than capable bird lens.