Some thoughts.I'll post my standard warning on all of the Canon built-in lens hood lens posts - it's all great and convenient until it's dented or otherwise damaged and Canon charges you $400+ to replace the hood. I learned the hard way, twice.
The 300f4 is bigger, but it has that sweet lens hood
ah i was wondering why they stopped doing this which i thought was a great invention!
anyway its a fantastic lens well balanced and very nice optically
the IS is very old noisy and clunky so don't think its broken when the IS makes a clunk sound when engaging
it even works ok with the 2x TC on 1D or 5Dmk3 bodies at f8 but its AF is very slow and will struggle to track anything
fast the focus limit switch helps
but honestly since getting the tamron 150-600 i am considering selling mine....
it wasnt a highly used lens for me anyway and often i would find 300 to short so was using a 1.4 or 2x tc with it
about the only thing the tamron doesnt have over this lens is f4 and the tamron is alot bigger and more bulky but its beats the pants off the 2x tc combo and its a zoom
on a 5D body the 300 f4L IS is a superbly well balanced lens and very easy to shoot with all day