This has been used in many builds over the years, but has long been out of production.
I’m not interested in using it, but I noticed when looking at something else that there seem to be many more of these around for sale these days, and at varying prices: for example, there are around 18 on EBay which range from A$60 to A$100 each. Some years ago, Jaycar (a local electronics supplier) was throwing them out on clearance for A$9.00!
Was it really so good and is there something strange going on that there seem to be more around compared with other discontinued products such as the RS28 series?
Those have been gone a long time and I don’t recall many people lamenting the loss, but I may be wrong. I had a pair - not sure where they went. I may have sold them when Iowa DIY was still in Grinnell. Looks like Hales used a version in one of their designs so there may be a market willing to pay those prices you found.
I had the D25AG back in the day. It was a nice tweeter with a well controlled metal breakup due to the phase shield, but nothing truly impressive. You mention the RS28, those were usher-like drivers on a whole higher performance level.