I'm always surprised that you rarely hear this work! I'm also surprised that this is Bach's only organ setting of this melody.
This piece is unique in several other ways: 1) it specifically calls for 3 manuals and pedal, 2) we know the occasion for which it was composed, and 3) we have Bach's own registration used.
This "Buxteudian" free fantasia was, according to Spitta, composed for the 1709 Reformation Festival at the St. Blasiuskirche in Muhlhausen. The organ had been rebuilt under Bach's (who was now stationed in Weimar) direction, and he was the first to demonstrate the "new" instrument.
The registration calls for the "Oberwerk Fagtto 16' " in the left hand and the "Sesquialtera" of the Brustpositif in the right. It also directs parts to be played on the Ruckpositif.
As I recall from someplace, Bach called the new Fagotto a "beautiful and wonderful reed," and described the Sesquialtera as being "sharp and lovely." I think it was Walter who noted the registrations that Bach used.
At any rate, I hope you like this "rare" piece!
By the way, I wanted to play this on the Trost organ, but I just couldn't make the articulation sound right, so, the performance wound up on Stade! ;-)