I know of three in the states - the prototype in Canada (variously owned by Bob Joles, Warren Deck, Morris Kainuma, Ian McIntosh, and now "Mandrake" on the BBS), one up in - I think - Oregon, and mine (formerly owned by Tommy Johnson).
Like a lot of big rotary horns, it can be very revealing. If you're not on your game, the horn can be a handful. I play avocationally now, and don't have the time to put in on the horn to really get the most out of it. This is why I've decided to sell mine. "Back in the day", I'd have never thought of letting it go.
Having said that, it's capable of producing quite an amazing sound. I posted some MP3s of mine in action with Tommy:
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Response/resistance - quick and typically "Mirafone-ish". Low register lights up in a hurry.
Intonation - solid, with a couple notes that need some pushing/pulling. Tommy would've never kept his as long as he did if it was out of tune.
Sound - huge, omnidirectional.
This isn't a horn for everyone. It doesn't exactly "play itself", but with a little effort, it can produce quite a special sound. Of course, all the yammering in the world won't substitute for some time on the axe. If you're ever out in Southern California, mine's always available for a toot...
...Dave