This was a fun. The main reason I took these pictures was to test the suitability of various lens/extender/aperature combinations for sports photography. The four combos were:

1. Canon EF 70-200mm f/4L USM with Canon EF 1.4X Telextender (98-280mm @ f/5.6)
   
2. Canon EF 300mm f/4L IS USM wide open (f/4.0)
   
3. Canon EF 300mm f/4L IS USM @ f/5.6
   
4. Canon EF 300mm f/4L IS USM with Canon EF 1.4X Telextender (420mm @ f/5.6)

The same Canon 30D was used for all lenses. The center focal point was used and I did not use a tripod or monopod. Upon initial review, it seems that achieving proper focus was possible with all combinations, with roughly the same consistency, with the possible exception of shooting with the 300mm lens wide open (f/4.0) without the extender. These images do not seem as consistently sharp as the others (nothing a little usm couldn't take care of, though). However, it is also quite clear that this combo also produced the best backround blur/subject isolation. Now I see why the pros shoot sports with f/2.8 300mm and 400mm L glass mounted on 1D's. That being said, I could definitely see a use for the 70-200mm with 1.4 extender combo in cases where 200mm is too little reach, 300mm is too much reach, and/or when attaining background blur isn't as essential (alpine skiing with snow background or waterskiing with water background, for example). Also, the 300mm lens with the extender is definitely a viable option when more reach is required. Finally, note that the images below were all edited, including cropping, sharpening, and resizing.