Hello everybody ive got a Canon 450D with a EF-S 55-250 F4-5.6 IS which i use mainly for motorsport, my question is would be worthwhile upgrading to the 100-400 F4-5.6 L IS USM to take motorsport pics.
what i would ask, is "do you need it?"
have a look at what you are currently using, and how it isnt doing the job you want it to.
is the focal range too short? is the image quality too low? is the aperture too small?
a lens like this is going to be VERY expensive, so its worth considering these things before you purchase.