I had the 70-300 DO for a while. As I have done with several lenses I had some doubts if I really wanted or not, I bought it used. It was very convenient with a size almost identical to the 24-105/4 (although it was f/4.5-5.6). Perhaps with the current state of DO lenses (which Canon seems to no longer be labeling as such, e.g. 600/11 and 800/11), I wonder if they could make a 70-300/4L with a DO group that is quite compact. Seems possible. That, I'd be sorely tempted to buy.
The 70-300 DO wasn't a bad lens, but it did have it's optical quirks as far as bokeh goes. I ended up selling it, for the same price I paid – a free, long-term rental is a great way to evaluate a lens! My track record with that approach has been good. Of the 5 lenses I bought used, I sold 4 of them and ended up net in the black after selling them (others were the 300/4L, 28-300L, and 24-105/4L although that last one I just replaced with a new one bought in a kit with a 5DII). The only one I kept after buying used was the MP-E 65 – a great lens, lots of fun, and I only paid $500 for it.
Here's a shot with the 70-300 DO, in the hopes that Canon brings out an f/4 L version in RF.
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Ribbit"
View attachment 205518
EOS 7D, EF 70-300mm f/4.5-5.6
DO IS USM @ 300mm, 1/500, f/6.3, ISO 640