The 70-300mm IS USM is actually a very good lens, both from my 4 year experience with it and based on the test report and reviews I cited above. It is vastly superior to the 75-300, and the 70-300 has been referred to as virtually L-glass in the reviews I cited (but admittedly not L construction).
Yes the 70-300 is good, 75-300 crap. The IS and lower cost and good IQ make it a good buy. I think the Tamron 70-300mm looks beefier and takes great photos though. And the VC is phenomenal.