I used an M5-1 long ago, the one with the trackball by the cursor keys:
Show Image
(http://i.imgur.com/AxtRcjQ.jpg)
Wasn't a fan, but then again I'm not a fan of trackballs in general.
Much preferred M13's when I needed a mouseless setup.
I think it's a love-it-or-hate-it thing, no in between. Some may buy it just for collectibility.
Oh wild! I never knew these existed either. I've been a fan of trackballs for years, but have been amazed at some of the odd placements of them. That layout near the arrow keys makes a lot more sense than upper right of the numpad.