|Canon Extender EF 2x II Specifications|
|Focal Length Magnification||2.0x|
|Lens Construction (groups)||5|
|Lens Construction (elements)||7|
|Maximum Diameter x Length||71.8mm x 57.9mm (2.8" x 2.3")|
We reviewed the Canon 400mm ƒ/5.6L USM back in 2008, which makes a good candidate for comparison with the 200mm ƒ/2.8L USM and 2x Extender attached. In terms of sharpness, the two are quite similar: the 400mm is slightly better at ƒ/5.6 and ƒ/16, but at ƒ/8 and ƒ/11, they're almost identical.
My advice is to use the Canon EF 2x II Extender to extend the usefulness of a very sharp lens that has another more primary purpose. Said another way, you generally will get better results from a 400mm lens than with a 200mm lens + the Canon EF 2x II Extender.
If you have a Canon system and think you need, or at least want a 2X Extender, then the new model is about as good as it gets. If, as I do, you already own the previous generation then I wouldn't be in a rush to replace it.