I've SEEN them, but I've never used one. The problem with them is that in order to play harp on a rack (which I used to do before my neck and double chin started getting in the way), you need to get the harp WAY into your mouth for the best tone. That, of course, is true no matter how you play the harp.

The racks that attach to the mic stand run the risk of wobbling around on you if the stand isn't super steady or if, more likely, the stage or performance area is less than level.

Jimmy Reed ("Bright Lights, Big City", "Big Boss Man" etc) played harp on a rack while he played guitar. So did Slim Harpo.

The thing is, however, that most people think "bluesman" and they think that these guys were always playing what we recognize as "blues harp". Not always so; not at all. Perhaps by the time they were recorded, there was a certain stylistic expectation, but the bulk of their performing careers were spent playing a variety of styles and not just a narrow idea of "blues".

Men like Harpo, Jimmy Reed, Jesse Fuller and others who used the rack, often played ragtime, ballads, work songs and other types of music with the harp in a rack while they played guitar or banjo or mandolin. The sound was more often closer to something you'd hear in vaudeville or in a saloon at the turn of the century rather than an urban blues sound that we would call "blues" today.

Consider this: men like Jesse Fuller or Jimmy Reed were not going around thinking of themselves as "innovators" or future legends; they were trying to EAT, and trying to entertain people at the same time. They would have been drawing on songs that were old, recognizable and hum-able back in THEIR day, so they wouldn't have been playing Chicago-style urban, Little-Walter-Jacobs phrases on their harps. Reed was sort of in the transition period between the "songster/music man" tradition and the later "bluesman", so there's some continuity, but by and large, harp in a rack back in the day was more melodic than it was "bluesy".

Remember... where there's a will, there's a way! Whatever creativity it takes to be able to make your sound, you can figure it out! Someone did all this stuff first, after all! :-)

Good luck!