I owned the 70-300 DO lens for a couple of years. While it wasn't a bad lens, the DO did take a bit of a toll on IQ, particularly in situations where lens flare was likely (strong light source striking the front element). Bokeh was a little awkward/choppy at times also. I understand that the 400/4 DO is a very good lens.
I'd like to see a refinement in the DO design that would produce an even better optical quality. The light weight and relatively small package makes a strong selling point for this technology.
Canon has a number of recent patents for new DO lens technology, but it seems to be exceedingly difficult to manufacture. It is based on small particles immersed in a resin compound. The resultant radial dispersion of particles can produce a superior lens element that varies its properties radially. The issue is getting the spacing of the particles right in a consistent way, and that may never happen. You can't just pour a powder in a vat of resin and mold a bunch of identical lens elements. Every one will be different.
They did not specifically mention this technology in the patent, but did talk about a resin compound, so its in there somewhere.
It may not be as easy as simply mixing "power" with resin an getting ideal results every time...but I would be willing to bet they could mix lightly charged particles with slightly varying charges in resin, and before it cools use a magnetic or electromagnetic field to disperse them properly.
Actually, Canon does give some information about their process in the patents for the diffractive lens.
I do not think that the particles are magnetic.
"DESCRIPTION OF THE EMBODIMENTS 
This embodiment provides an optical system with a radial gradient-index optical element having a refractive index that changes in a direction perpendicular to an optical axis in a medium. In comparison with an axial gradient-index optical element having a refractive index that changes in an optical axis direction in the medium, the medium of the radial gradient-index optical element has a refractive power caused by refractive index distribution, So the radial gradient-index optical element has an advantage of correcting the chromatic aberration by setting a proper refractive index distribution. 
In the gradient-index optical element, when a refractive index distribution for each wavelength can be independently controlled, the chromatic aberrations for the d-line, the C-line, the F-line, and the g-line can be simultaneously corrected. However, it is difficult to independently control a refractive index distribution for each wavelength when the gradient-index optical element is actually produced. 
Accordingly, this embodiment properly controls a wavelength dispersion characteristic of the gradient-index optical element, and thereby realizes an optical element that has an effect of correcting the chromatic aberration. 
An ion exchange method, a sol-gel method, and three-dimensional printing are known as a manufacturing method of a gradient-index optical element. These methods can distribute a refractive index by gradually changing a composition ratio of the gradient-index optical element for each location. In this gradient-index optical element, this embodiment properly sets a refractive index distribution so that the following conditional expressions can be satisfied, and the chromatic aberrations for four wavelengths can be well corrected. "
However, as with all patents, its easier said than done, so I'm not overly optimistic about seeing a low cost DO implementation soon.