I also have the 70-300 DO.
I like it, because it's small and inconspicuous - I can't imagine myself walking on the streets on holiday with a big, fat, heavy, conspicuous white lens...
The image quality of my 70-300 DO is good but not top - I don't have an L telezoom lens, but an L would undoubtedly be better than the 70-300 DO. Especially at the long end and wide open (300mm, f/5.6) it's not really super sharp.
I've recently made some concert photos
(outside, it was getting dark). Note, the lack of sharpness is not because the lens is soft, but because of motion blur. The DO is not a very fast lens, with its max. aperture of f/5.6 at the long end. Because of that it's less well suited for low lighting, indoor events - it's best when using it outside with a lot of light.
The 2-mode IS on this lens works great - it's better than the IS on my 28-135.