I bought a used 70-300mm DO lens and used it for a few months. I loved the compact size (same size as a 24-105L), the IQ was ok with a little extra TLC in post (boost contrast and sharpness a bit more than normal). I didn't like the horrible zoom creep, and as I said, the IQ was ok - but not great. I also didn't like the bokeh. I eventually sold the lens (for the same amount I paid for it) - a free long-term rental as a try-out. That was before the 70-300L came out - I'd definitely prefer the L to the DO lens.
Here are a couple of shots:
EOS 7D, EF 70-300mm f/4.5-5.6 DO
IS USM @ 300mm, 1/400 s, f/5.6, ISO 500
EOS 7D, EF 70-300mm f/4.5-5.6 DO IS USM @ 300mm, 1/500, f/6.3, ISO 640