Yes I was going to send my lens back after 3 days because I thought it was a bad copy.
Aye, there's a moral there!
Back in my Nikon days, it took me six months before I started seeing the results I was looking for from the Sigma 80-400mm OS lens on my D70: different lens, same kind of issue - nothing seemed as sharp as I expected.
Then one day, BANG! It all started working out.
When I subsequently came to Canon about a year ago, it took me approximately one afternoon to get my head round the 100-400mm.
Even so, it seems to me that I'm still improving, and even after a year I regularly get pictures that make me go "Wow! That's better than I expected!"
To put it another way: I don't believe I've got a super-sharp 100-400 (though it's plenty good enough for me) and yet I'm sure that I haven't reached its full potential yet - there's better to come as I get better at using it.