I had a 1.4X V I and 2X V II. I upgraded both to V III and was very happy. The 1.4 went from acceptable to I swear I owned a 600mm prime (well 560, when attached to my 400mm f2.8 IS) The 2X was an improvement, but still not sharp like a bare lens, or the 1.4.
I am thinking of selling my 2X V III, because I don't see a ton of difference in a 800mm shot, and crop from 560mm. I keep hoping to run into someone with a VIII 2X to compare mine to. Maybe mine is off a little.