I just sold my 500 F4 I and purchased a 500F4 II. I also got a 1.4x III to go with it. I have used it three times for birds. If you have never used one of Canon's 500 Superteles, you are going to be blown away by the sharpness, and quick focusing. In my brief experience, the new version isn't much sharper than the excellent original version, but the autofocus is faster and the IS is much better. Although the new lens is much lighter than the original, you will need a strong tripod with a gimbal head to use it to photograph a race. Let us know what you think after the race?