I still own my 2nd 70-300mm DO.
When it first was launched, it was in a class by itself in that it was not only compact, but the build and features of this lens were light years ahead of any other 7x-300mm lens.
In essence it was the FIRST 70-300mm "L" in build quality, AF and IS etc..
It's AF is much faster than the then current 100-400mm f/5.6L IS (1st version)
It's IS was also far better.
It does not have the 100-400mm's contrast though, but in most cases, this could be fixed in post. The images also benefit from a slightly different sharpening process than most other lenses.
Since the DO, there are other options. The "L" has been mentioned, and SIGMA's newest, the 100-400mm C is also worth mentioning, though not at all as compact as the DO, it is very small and light for a 400mm zoom.
We just took that SIGMA across Botswana these past two weeks,. and the results look fantastic. We did not bring the DO
Both the Canon "L" and the new Sigma are in some ways better than the DO, BUT of the three, only the DO is going to give you 70-300mm in a truly small stealthy lens.
I have no hands on experience with the newest "kit" 7x-300mm,. historically all the Canon ones have been pretty bad. If they have made it comparable to the 50-250STM, then it woudl truly be another to consider.