I haven't played one of those but I did play something similar a couple of weeks back - just the one 15" speaker facing upwards behind two 10" though

I'm not really sure on the supposed benefits of the design and I'm not surprised such things didn't catch on.
yes i have. the week-end i purchased my HW1 Jazz bass in '04. Murphy's Music, my local shop, had one of these hooked up to an Ampeg SVT-3PRO amp. i sat in front that thing for about 2 1/2 hours. using it, to help me choose my new bass. it sounded great. the design was something i had never seen before, and still don't understand, but it did sound good. if it's close enough to drive to, i say check it out.
take your amp, and see. i'd give $250 or $300 just to own it. tell us what you find out, sonnyboy!!
If anyone is considering buying it, by the way, that isn't the seller's photo. The picture is the same as one of the top google image results.