the 70-300 DO has good and bad points. I have seen great results from 2 of them (my own, and a friend's, - plus Michael's less directly). As you have seen, it attracts mixed reviews. I believe there are 4 main causes for the "poor" ones -
1) it lacks resolution compared to some heavier lenses when tested for resolution at 200-300mm around f5.6
2) its point spread function is different from most lenses, so it requires different sharpening for best results - I guess many miss that, or do not understand the difference between sharpness and resolution
3) it is fast to use, and therefore, easy to make mistakes (focus, shake): a heavier lens would be more often used on a tripod
4) probably there is the usual production spread, damage etc. that leads to some poor examples.
On the last point, because every example I have seen results from gives pretty good quality, I find it hard to believe there are "special" ones. I guess mine and my friend's are just as good as Michael's. Of course there will be poor ones, when there are so many tolerances involved, each of which can make performance worse if out of spec. It is, however, very unlikely that errors come together in such a way as to produce an unusually good example.
Why do I bother to say this?
My DO has revolutionised my photography, adding a new dimension, on returning from photographic outings, I often find that ~70% of images have been taken with it.
I would have said that for zoo trips etc., in places where it sounds that light will often be plentiful, the DO is OK. Depends a bit what else you want it for. (I always use my 70-200 f4 with a tripod, sometimes I use the DO handheld, what freedom!)
The 70-200 f4 with 1.4x (which I have) is a touch better at f5.6 than the DO, but if handheld you can set the DO to f8, similar sharpness, and still have less shake, if you can tolerate the extra depth of field.
Have fun either way.