In fact I was less than happy with the DO, especially considering the price. (Note the past tense - I recently sold it for the same reason!)
The problem with the lens (or my copy, anyway) is that it doesn't give consistent results: Some pictures were excellent in sharpness and colours, others very bad (not only because of the person behind it!) - I'm not alone in this criticism - take a look at http://www.fredmiranda.com/reviews/showproduct.php?product=218&sort=7&cat=27&page=3 .
Instead I've bought a 100-400L, which only costs a ca 10% more (Danish prices), but is MUCH more versatile in terms of picture quality (but not in size and weight where, admittedly, the DO excells).
My advice to you would be to return the DO and get another type, perhaps the newer 75-300 IS, unless your're absolutely sure that your copy of the DO is one of the good ones!