I have also owned the 70-300mm DO. For its original price it is not a great lens at all, in fact I skipped it all together. Then some years later I bought a great used version for £450 which is less than a 1/4 the original value. It performs IMO similar to the 24-105mm but is a little softer and bokeh isn't as nice and needs more PP to get the images to look as you would want. The fall off is also interesting, the Bokeh is also a little oniony, but in practice I found the Bokeh onion ring effect difficult to produce, the fall off is much more noticeable quite cluttered not smooth. It also produces the halo effect in high contrast situations.
But once you get over this fact and get one for a good price and do the extra work it is a marvellous lens. If you are traveling it will be a great companion. Compressed it is the same size as the 24-105mm, it is heavy for its size (720g) but it is tiny and black which is another great strength. I wouldn't hesitate to buy the 70-300mm L but going into countries that aren't quite as developed or fortunate and waving high end white lenses around is asking for trouble. The F4.5-5.6 is a little annoying but with a FF DSLR its not a bad compromise.
I actually really liked my copy but sold it as I needed more light gathering capability for events. Wish I hadn't sold it, it fills a niche and isn't the best performer but for its size weight nothing can compete with it. With IS 70-300mm range and its size it really is a great choice.
I have been looking for another copy as I am going traveling for 6 months in South america and US in October. Been looking at lots of sample images on FF as I owned the lens with a crop camera.Untitled
by Adam Perfect
, on Flickr
If you have a look at the image closely or go on Flickr and download the full res the fall off at the bottom of the frame is interesting. But it is sharp where it needs to be, having a lens that is easily carryable will get much nicer shots than a lens left at home because of its size/weight.