I had one, sold it. Best thing about it is the small size - same as the 24-105L. IQ is just ok, needs work on contrast and sharpness in post (although the DxO module handles that well), has an odd bokeh that post processing can't fix (you can see it in the foreground of the image below).
EOS 7D, EF 70-300mm f/4.5-5.6 DO
IS USM @ 300mm, 1/500, f/6.3, ISO 640
Zoom creep is horrible - once with the lens fully extended, when tracking a bird flying overhead, the barrel slammed back in so hard I almost gave myself a black eye with the eyecup.
I do see them a lot on Craigslist, and they don't move very well. So if you're going to buy it, I'd try to pay no more that $700 so you can resell it without much loss (and hopefully none).
I have the 70-300L now, it's really an excellent lens! Despite being a bigger than the DO version, I far prefer it.