I had one on my 8x10 and was generally pleased with its performance. Decently sharp at 13", good at 20" with a #15 yellow filter to cut chromatic aberration. I found the 20" conversion most useful. At 25" it was maybe not as sharp, but my old camera was so shaky I can't honestly say, and, at 25", it's a big pain to focus and use camera movements. Unless you're really tall, your arms just aren't long enough . . .
I think I paid $350 for mine, maybe should have paid $300, but I got my money's worth out of it. They don't seem to be particularly rare -- I've seen several come up for sale over the past couple of years.
Peter Gomena