The Canon 70 - 300 L lens is tough and water proof, more or less, but the Tamron 70-300 is an excellent lens too. Knowing how similar they are in performance, I may have invested in a longer lens and kept using the Tamron in it's class.
So if you own a Tamron 70-300, be sure to respect it. A very good lens indeed.
Now I think I need a 600mm lens... or thereabouts. any ideas? TGT