I tried one of these recently; it was an interesting experience.
Firstly, I am used to playing slides of different lengths, tenor, alto and descant, so I switched to descant mode and coped very well with the half length slide and positions.
The slide was like any other plastic trombone slide; it needed work.
The biggest problem was the fact that high F
was a very strange, unfocused note. It was fine in #4 but not in 1st.
The other problem for me was the ergonomics of the thing. The double slide is built so that in cross section the slide tubes are at the angles of a rhombus instead of a rectangle. This reduces the room for the fingers of the left hand and felt very uncomfortable. I tried to hold it with all the left hand fingers going over to the mouthpiece receiver but it didn't help.
Would I use one? Possibly.
Would I recommend one? Only if the player has tried to play a descant trombone already.
Would I buy one? I am still thinking about it. The sensible half of my brain says "Don't be stupid" but the crazy half says "Why not. Have some fun" Problem is that the crazy half of my brain is twice the size of the sensible half and growing with advancing years.