I've never played one (due to the obvious rarity,) but I used to have a close professional relationship with someone at DR Strings, the company that developed Hellborg's signature "single-wrap" strings.
A lifelong DR player, I pressed him for details. Specifically, his personal impressions. "They're... twangy."
Eventually he advised me against stocking them. As always, we had a good time shootin' the poopie, and I asked him about the signature bass. I could practically hear his eyes dart around the room, before he confessed, "It's the most ridiculous goddamn thing I've ever seen." Looking at the design/reading the specs, I'm somewhat inclined to agree.
But, of course, dif'rent strokes for dif'rent folks. I like a big, fat tone. Hellborg seems to want a nasal, treble-heavy tone with emphasis on overtones over fundamentals. That may be your thing. I'd prefer to sound like Jamerson rather than Jaco. I also prefer red wine over white. Your mileage may vary, but one thing that can be said is that it's a fairly radical instrument, and not for the faint of heart.