I was completely unfamiliar with this gorgeous Adagio before undertaking the Brandenburg Duets transcription project and really hope that now it will gain more appreciation in a solo piano transcription. It has become my absolutely favorite solo Bach arrangement to play. The theme, structured upon descending melismatic patterns, sounds very exotic, somewhat Arabic and mesmerizing. Imagining the first 3 measures of the melody coming out of a snake charmer’s flute doesn’t seem far-fetched at all. The atmospheric accompaniment of “cushions” of chords helps establish the mood and sounds good even at a very slow tempo.
For inspiration and reference, it’s always useful to listen to recordings: there are plenty of orchestral ones out there and my piano duet version of is also available.