There are few online reviews of this old, now discontinued, lens. They were favorable but complained of low-contrast images and a high price tag (about $1,150 new). These two factors are no longer relevant - software can increase contrast, and the lens sells for about $500 on the used market.
Canon's diffractive optics proof of concept
The Canon 70-300mm DO was originally expensive because it was one of the first tests of 'diffractive optics' in a Canon lens. This used a saw-toothed fresnel lens to magnify an image while keeping the un-zoomed length of the lens to a minimum - at the expense of some contrast and a small maximum aperture (f4.5-5.6).
Historically, the lens competed against the better, and larger Canon EF 70-200mm f/4L IS (which is compatible with Canon's extenders. unlike the DO), along with the 50% cheaper, and larger, EF 70-300mm f/4-5.6 IS USM. At the original selling price, the DO was a tough sell - only people who valued its short length as a compact travel lens would pony up the cash.
But then the DO lens got older. The price came down. I also think the mediocre reviews (based on the high price tag) contributed to the massive reduction in the used price of the lens. The only other reservation you might have about this lens is the variable f4.5-5.6 aperture, but again, modern cameras and software compensate for this with excellent high-ISO quality that was unheard of when this lens was first released.
The best travel lens set for Canon
So when the DO is viewed in today's context - a low priced compact lens - it is a no-brainer for a small travel kit. I pair it with the 40mm pancake lens when taking a digital camera on trips. The 40mm stays on the camera and the DO comes out when the extra reach is needed. If the DO is on the camera, the 40mm can be in a trouser pocket.
You can see some examples of images taken with this lens (including many of my own) through the Instagram tag #canon70300doisusm.
For those of you on the fence about getting an old 70-300mm DO lens, there has never been a better time to get one.