I have been using the 70-200 f2.8 II lens, which is exceptional, but it just wasnt long enough to cover kids on the Soccer field. So I found a mint UX dated 100-400 on Ebay for $1250. I was a bit reluctant considering the mixed reviews, but after a weekend of shooting I am more than impressed.
I have been living mostly on the far end of the lens, shooting wide open at 400mm and the results are better than expected. Very sharp and contrasty images, the focus speed has not been an issue, and frankly Im getting more shots in focus than the 70-200 (which is likely more to do with the field of view).
The push pull zoom is a breeze, despite what others have said. Quite frankly, for sports its a better option imo.
For the money, this is a spectacular choice.
The only negative is the slow speed, which hasn't been a major issue as Im shooting with a 5D3, so I just crank up the iso, but if a faster version came along, I would defiantly trade this one in assuming it was as sharp as this model.