Even though I shoot a lot of sports, I won't get the lens either. Admittedly I already own the 200 f/2, 300 f/2.8 and 400 f/2.8, but I shoot wider than f/4 a ton and this isn't very useful for me. In fact, for indoor volleyball and basketball I set aperture range from f/2 to f/4 with ISO safety shift on (or if I'm using the 70-200 lens, f/2.8 to f/4). I prefer the faster shutter speed in exchange for opening up wider.
However, I can definitely see the utility in this lens, and if I didn't already have the 300 and 400, I probably would use it for football and soccer, as long as they were day games or well-lit venues. My problem is that my night stuff is NOT well-lit, so to keep a fast shutter I need wider than f/4 probably.
I think this lens would be great for well-lit sports, and wildlife photography, and it'll add a ton of convenience over the longer primes.