I had version I for a year and a half and just bought vIII. Noticably better image quality with my 400mm f2.8 IS (version I), especially in the corners. I've also used it with my 300mm f5 IS, but can't says as I've really looked at photos side by side like I did with the 400.
Before buying I used the comparison tool at The Digital picture to get an idea if i'd see an improvement. They showed an improvement with both the 2X and 1.4X version IIIs, and that is what i found when I bought them.