The Hohner Special 20 is a harmonica/harp that comes in all of the Western keys. The one that I've used was in the standard key of C, which is probably the Special 20 that you'll see the most of out there. For a diatonic harmonica, this kind is definitely up there with the best of them. They seem to be quite popular, although I don't personally own one. I've got other harmonicas at home, but the Special 20 is one that I've used only on the other side of the glass as a recording engineer after a client brought it in to record with. While I can't speak to it's playability, the sounds of the Special 20 is superb. Of course it helped that the player was top notch, but I'd be hard pressed to hear another harmonica that surpassed the sound of this one if I could try it with the same player. Since I don't own this I can't shoot it out against the Hohner harmonicas that I have, but I do think it's a bit better than the BluesBand Hohner
harmonica I have. I don't believe that they sell the Special 20's in a set, but you can definitely buy one for each key and even in a 7th cross harp position. The price of the Special 20 harmonicas is a bit more expensive than your average harmonica, as it's possible to get a harmonica for next to nothing, but to get one of quality like this one, it's going to cost a little bit more, which really isn't all that much since harmonicas are pretty cheap in general. Unless you're a pretty experienced player, the Special 20 might not be necessary, but if you're looking for a quality harmonica to record with and are an experienced player, the Hohner Special 20 definitely should be considered.