When the section of the A61 through Chesterfield was resurfaced a few years ago, these were installed in place of cateyes and the usual rectangular edge markers.
From a cyclists point of view, they are NOT GOODtm. They are very slippery and cause your wheel to jump sideways if you hit them. They are marginally worse than the old cateyes. Fortunately, cateyes are usually only used in the center of the road and not on the edge. These things are being used on the edge!
From a visibility point of view, they are no better than the rectangular reflectors or cateyes.
They are also much more fragile than cateyes. After about 2 years of use, a good number have been destroyed or chipped. Many more have become grubby and coated in dirt. If they become chipped, dirty or crazed, their visibility dies but they remain a hazard.
All in all, a poor idea IMHO...