I have the genuine Canon hoods but never use them as they are not much use for zooms. I am amazed at the number of people I see still using them on expensive lenses.
I use the rubber Hama hoods (buy from 7dayshop) on my zooms. They are cheaper than the Canon, pop out to 4 different positions effectively zooming with your lens and so give much better performance at the long end. If you think about it a fixed hood for a 70-300 has to be fixed to allow 70mm coverage, so you get nothing like the stray light protection you would from a fixed 300mm hood at the 300mm end of the zoom. Clearly, depending where the sun is this might or might not make any difference, but why spend £100's on a lens then risk compromising the performance for the sake of a £15 lens hood and a bit of thought.
The only possible disadvantage is at the wide end the petal shape of the original hood is slightly more efficient, but this is lost as soon as you zoom a little. Also the rubber gives the end of your lens much better protection against everyday knocks than a rigid hood.