I once had the Canon EF 75-300 f4-5.6 lens, albeit not the lll version, and I have agree that it is a very mediocre lens.
It suffered quite badly from 'lens creep'.
I was going to replace it with the Canon EF 70-300 f/4-5.6 IS USM.
However, after reading several reviews, I went for the Tamron SP 70-300 f/4-5.6 Di VC USD, which came out top in this class (price range) of 70-300 lenses.
I can say, that I have been very impressed with the lens, both in terms of results and build quality.
Is also cheaper than the Canon, by around £100.
Don't confuse this Tamron lens with its cheaper brother, the Tamron AF 70-300, they are a very different 'animal', and the AF 70-300 is not in the 'same league' at all.
Dave