It took me nearly a year to find a used 70-300L at a price I was willing to pay. Used versions come up for sale very rarely. If you look at the specs, it doesn't seem like much, but in reality, on the camera, it's an outstanding lens. I suspect that the people who buy it simply don't want to part with it. I've owned and loved a 70-200 f4 IS for many years, but after renting the 70-300L for a sports event 18 months ago, I decided it was a lens I needed to own. I've been using the two side by side for the last few months. If I could afford to keep both in the long term, I would, but finances require that I sell one of them, and I've chosen to sell the f4 IS - the 70-300L is THAT good. It's on a par with the f4 IS in every respect: IQ, sharpness, IS, focusing speed, etc., but it's the extra 100 mm that tips the scales for me. That, and the compact format.
So my much-loved f4 IS is now on the market, and I have a feeling that the 70-300L will be in my bag for years to come. It's just a wonderful piece of equipment.